Oxygen consumption, body temperature, respiratory frequency and respiratory water loss of White Leghorn x Rhode Island hens were measured for short periods at six air temperatures between 2 and 32 degrees C. The hens were kept between tests in an open shed. The experiments were carried out over 3 years. 2. The upper critical temperature (Tcu) was estimated by the air temperature at which: 1, respiratory frequency increased above 60 respirations/min and 2, body temperature increased by 0.3 degrees C above that at the lower critical temperature. These responses to the test temperatures were examined as a function of the acclimatisation temperature (Ta) represented by the mean daily temperature during experimental periods. 3. A seasonal change in Tcu was observed, which correlated with Ta(r = 0.836). The seasonal 10 degrees C change in the Ta brought about a 3 degrees C change in Tcu, compared with an 8.5 degrees C change in the lower critical temperature. 4. Thermoneutral temperature zone decreased with increasing Ta; the two critical temperatures tended to merge at a Ta of 32 degrees C. The latter probably represents an upper limit for acclimatisation to heat.
