1. The resting heart rate of cocks and hens was measured in varying degrees of heat stress, and during acclimatization to heat.2. When the rectal temperature was below 110°F. (43·3° C), the heart rate varied inversely with the severity of the climate up to an air temperature around 99° F. (37·2° C).3. At air temperatures of 99° F. (37·2° C.) and above the heart rate was nearly constant over a wide range of rectal temperatures below 110° F. (43·3° C). It averaged about 50 beats/min. less than in an equable climate.4. Above 110° F. (43·3° C.) there was an increase of about 30 beats/min./° F. (17 beats/min./° C.) rise in rectal temperature. The rate equalled that found in an equable climate, when the rectal temperature reached 111–112° F. (43·9–44·4° C). Over this zone of body temperature the fowls were often agitated.5. In severe heat stress there was a slight fall in the heart rate on acclimatization, but this was small compared with the effect of climate.6. In severe heat stress the heart rate during the day was slightly higher than that at night.7. The relation of these findings to the mechanism of temperature regulation is discussed.

SummaryEnergy balances have been determined, using the comparative slaughter procedure, over 3-week periods on groups of laying hens kept at ambient temperatures of 7·2, 15·6, 23·9, 29·4 and 35 °C.Energy intake declined as the environment became warmer (kcal ME/kg¾/day = 203· 1·13°C); heat production, as measured by the difference between energy intake and energy retention, also declined with increasing ambient temperature (kcal/kg¾/day = 151 – 1·11°C). There was a linear relationship between heat production and ambient temperature with no thermoneutral zone or critical temperature.The energy available for egg production remained almost constant at 50 kcal/kg¾/day equivalent to a rate of egg production of 82% at each ambient temperature.

Heavy infections of E. acervulina result in a depression in body weight of the host, but anorexia induced by the disease is not the only factor responsible for this, since normal birds starved to the same extent did not lose as much weight. It was shown that the absorption of l-histidine and glucose from the infected part of the small intestine was depressed and may be a contributory factor to this weight loss; restricted feed intake alone tended to increase, rather than decrease, absorption.