. Estrogen attenuates the cardiovascular and ventilatory responses to central command in cats. J Appl Physiol 92: 1635-1641, 2002; 10.1152/japplphysiol.00981. 2001.-Static exercise is well known to increase heart rate, arterial blood pressure, and ventilation. These increases appear to be less in women than in men, a difference that has been attributed to an effect of estrogen on neuronal function. In decerebrate male cats, we examined the effect of estrogen (17␤-estradiol; 0.001, 0.01, 0.1, and 1.0 g/kg iv) on the cardiovascular and ventilatory responses to central command and the exercise pressor reflex, the two neural mechanisms responsible for evoking the autonomic and ventilatory responses to exercise. We found that 17␤-estradiol, in each of the three doses tested, attenuated the pressor, cardioaccelerator, and phrenic nerve responses to electrical stimulation of the mesencephalic locomotor region (i.e., central command). In contrast, none of the doses of 17␤-estradiol had any effect on the pressor, cardioaccelerator, and ventilatory responses to static contraction or stretch of the triceps surae muscles. We conclude that, in decerebrate male cats, estrogen injected intravenously attenuates cardiovascular and ventilatory responses to central command but has no effect on responses to the exercise pressor reflex. exercise pressor reflex; heart rate; arterial blood pressure; tendon stretch; muscle afferents TWO NEURAL MECHANISMS, namely, central command and the exercise pressor reflex, are widely believed to evoke the cardiovascular and ventilatory responses to exercise (17, 36). Central command is defined as the parallel activation of neural circuits in the brain stem and spinal cord that control motor, ventilatory, and cardiovascular function. Central command does not require feedback from the periphery (10). The exercise pressor reflex arises from the stimulation of group III and IV muscle afferents. This reflex is believed to be evoked by mechanical and metabolic factors in muscles while they are contracting (4,18,24).The cardiovascular responses to exercise have been reported to be less in premenopausal women than in postmenopausal women or men (11,12). The difference has been attributed to estrogen, a hormone that is well known to decrease vascular resistance (28). In addition, estrogen administration to postmenopausal women with mild hypertension has been reported to attenuate the pressor response to exercise (27). These studies, however, provided no direct evidence that estrogen exerted its effect on central command or the exercise pressor reflex. Consequently, we were prompted to investigate this issue in decerebrate cats. Using this preparation, we were able to examine the effect of estrogen on the cardiovascular and ventilatory responses to central command and to the exercise pressor reflex, each of which was evoked separately. METHODSGeneral. Adult male cats were anesthetized with a mixture of 5% halothane and oxygen. Catheters were placed in the right jugular vein and common carotid artery for de...