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Department of Physiology, University of Tasmania, G.P.O. Box Number 252 C, Hobart, Tasmania, 7001, Australia

1. The respiratory responses to CO₂ inhalation were measured in New Zealand White rabbits. Prior to testing, the rabbits either received drinking water ad libitum (C), or were subjected to 72 hr water deprivation (WD). The rabbits were tested in a climatic chamber at either 20 °C, (C20, WD20) or 30 °C (C30, WD30).

2. CO₂ exposure caused increases in both tidal volume (V_T) and minute volume (V_E). The direction and magnitude of changes in respiratory frequency (f), however, were dependent upon treatment.

3. Linear regressions were drawn for f vs.% CO₂ in inhaled gas. Treatment C20 showed zero slope, treatment C30 a large negative slope, and treatments WD20 and WD30 a significant positive slope.

4. The slopes of the regression lines obtained from plotting f vs.% CO₂ were plotted against their intercepts for each rabbit under each treatment condition. This yielded a highly significant linear regression from which it could be concluded that the frequency response to CO₂ exposure is dependent upon the initial frequency when the rabbits are breathing air.

5. It is suggested that frequency is shifted during hypercapnia towards an optimal value which is dependent upon the existing% CO₂ in the inspired gas.

6. Comparison of the results obtained for the rabbit with results reported for the conscious dog and sheep suggest that a similar relationship may hold for these species.