SUMMARY1. Intravenous infusions of adrenaline and noradrenaline in doses averaging 0.8 ,tg/kg . min increased the respiratory minute volume of anaesthetized cats breathing room air. The mean increase in respiratory minute volume was 14 % during adrenaline infusion and 8 % during noradrenaline infusion.2. In a small group of decerebrate cats infusions of adrenaline and noradrenaline increased ventilation by 19 and 27 % respectively.3. Intravenous catecholamine infusions also increased the respiratory responses of anaesthetized animals to the inhalation of 5 % or 10% 02 in N2 and to the inhalation of 5 % C02 in air.4. Adrenaline and noradrenaline infusions had no significant effect on the ventilation of animals breathing 100% 02, nor did they significantly alter the respiratory response to the inhalation of 5 % C02 in 02.5. After section of the carotid sinus and aortic nerves, a blood-pressure compensator being used to minimize changes in arterial pressure, catecholamines had no effect on the respiration of cats breathing air.6. An increase in carotid body chemoreceptor discharge accompanied the increase in ventilation during catecholamine infusion.7. Intravenous catecholamine infusions still produced an increase in ventilation and carotid body chemoreceptor discharge after both aortic nerves and both cervical sympathetic nerves had been cut.8. Intra-arterial infusions into one carotid artery of 0 2 /tg/kg. min of adrenaline or 0 1 ,tg/kg. min of noradrenaline led to mean increases in respiratory minute volume of 9 9 and 11.5% respectively. No increase occurred after section of the corresponding carotid sinus nerve. Such infusions also evoked an increase in carotid body chemoreceptor discharge.9. It is concluded that the hyperpnoea produced by adrenaline and I1Phy. I97

The complete dissociation curve of carboxyhaemoglobin has not been studied since Douglas et al. (1912), using the more accurate and convenient methods available at the present time. COHb dissociation curves have therefore been determined in the absence of oxygen, at three different C02 pressures, for the blood of three normal subjects. These curves have been compared with the corresponding 02Hb curves. METHODSFor each experiment 20-25 ml. of venous blood was freshly drawn into a syringe, the dead space of which was filled with heparin solution.Deoxygenation of the blood. Before determining the carboxyhaemoglobin dissociation curves, the blood was deoxygenated as completely as possible, to exclude the effect of any residual oxyhaemoglobin on the uptake of carbon monoxide. 17 ml. of the blood was introduced into a 410 ml. tonometer which was evacuated to a pressure of 50 mm Hg. Further lowering of the pressure was found to cause boiling of the blood and undue haemoconcentration. Nitrogen was then admitted to the tonometer to restore the pressure to atmospheric. This was followed by a second evacuation and restoration of the pressure with nitrogen, after which the tonometer was rotated for 5 min in a water-bath at 370 C. The whole cycle was repeated four times making a total of eight evacuations and four 5 min periods of equilibration with nitrogen at 370 C. The

SUMMARY1. The reflex responses to baroreceptor stimulation have been compared in eight pregnant and eight non-pregnant anaesthetized female rabbits.2. The vascularly isolated, innervated carotid sinus was exposed for 30 sec to a series of non-pulsatile pressures ranging from 30 to 230 mmHg. The contralateral sinus nerve and both aortic nerves were cut. Systemic arterial pressure and heart rate were measured at each sinus pressure.3. The range of arterial pressure change which could be evoked from the isolated innervated sinus was less in the pregnant than in the non-pregnant rabbits. Mean changes were 98 and 61 mmHg respectively. On the other hand changes in heart rate were similar in the two groups (45 and 43 beats/ min respectively).4. The smaller blood pressure response in the pregnant animals resulted from a lesser rise in systemic arterial pressure at low levels of sinus pressure. At high sinus pressures the blood pressure fell to a similar level in both groups of animals.5. Pressure on the great vessels by the gravid uterus was not a factor since there was no consistent difference between the responses obtained with the rabbit lying on its back or on its side.6. Mechanisms which might be responsible for the difference found are discussed.

SUMMARY1. The reflex cardiovascular effects of changes in pressure within the isolated carotid sinus have been examined in sixteen anaesthetized pregnant rabbits.2. Comparison of the mean results at sinus pressures of 40 and 200 mmHg showed that heart rate fell by 32-1 beats min-' and arterial pressure was reduced by 48-2 mmHg. Cardiac output, measured by thermal dilution, fell by 25-7 ml. min-'.kg-1 and total peripheral resistance was reduced by 0-292 mmHg.ml.-' min.kg.3. The corresponding changes previously reported in a group of seventeen non-pregnant female rabbits were a similar fall in heart rate of 34.5 beats min-' but significantly greater reductions in arterial pressure of 69-8 mmHg and in total peripheral resistance of 0X432 mmHg. ml.-' min. kg. However, the fall in cardiac output of 12-6 ml. min-' . kg-1 was significantly smaller.4. These observations show that the smaller blood pressure response of the pregnant rabbit to alterations in sinus pressure is due to a reduced change in total peripheral resistance.

3. During hypothalamic stimulation the reflex fall in blood pressure resulting from a rise in sinus pressure was found to be undiminished whether sinus pressure was raised at the onset or at the 10th sec of hypothalamic stimulation.4. By contrast, in at least half the cats in which a reflex bradyeardia could be evoked from the sinus, this bradycardia was largely if not completely suppressed during hypothalamic stimulation. This suppression of reflex bradycardia occurred when sinus pressure was raised at the onset as well as at the 10th sec of stimulation.5. During the first 5 sec after hypothalamic stimulation the hypotensive response to an increase in carotid sinus pressure was much reduced; on the other hand the reduction in heart rate was exaggerated, sometimes to a very marked degree.6. The results suggest that stimulation of the hypothalamic defence area can modify baroreceptor reflexes and that this modification can include selective alterations in the various components of the reflex response.