1. Rabbits were clipped and exposed in turn to four environmental conditions: control (C), cold exposure (CE), water deprivation (WD) and water deprivation and cold exposure together (WD/CE). 2. Following each type of treatment, the rabbits were exposed for 1 hr to an ambient temperature (Ta) of 35 degrees C. During this time, respiratory frequency (RF), rectal temperature (Tre), activity and oxygen consumption (V02) were recorded. 3. It was found that under both cold exposure and water deprivation conditions, the mean respiratory frequency during the first 30 min of heat exposure was reduced when compared with controls. This was associated with a delay in the onset of thermal tachypnoea. Under conditions of water deprivation and cold exposure together, the mean respiratory frequency was further reduced and the length of the delay was increased. 4. Previous cold exposure led to an increase in the V02 measured at 35 degrees C, whereas the V02, after water deprivation and water deprivation and cold exposure together were not significantly different from the control. 5. Neither the initial Tre nor the change in the Tre during the course of the heat exposure were significantly different from the controls under any of the experimental conditions. 6. It is concluded that both water deprivation and previous cold exposure cause a block to panting in the heat and that the blocking mechanisms involved are closely interrelated. It is also concluded that neither the metabolic rate of the animal nor its initial or final Tre are important factors in determing the degree to which thermal tachypnoea is inhibited.
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