An experiment was carried out to study the influence of a low ambient temperature on the development of homeothermy in baby chicks. Three groups of male chicks, White Leghorn (WL), Barred Plymouth Rock (BPR) and BPR×WL, were reared for four weeks after hatching, at ambient temperatures of 20 to 24℃ and of 30 to 33℃ with a humidity of 65 to 75 per cent. Chicks of 44 to 47 were allotted to each group. Mortality, rectal temperature and body weight were recorded and the thyroidal release rate of I^131 was studied during 12 to 19 days old. The results obtained are as follows: 1) High mortality in the 20-24℃ experiment was observed, i.e., 80.4 per cent in the WL, 55.6 percent in the BPR×WL and 42.6 percent in the BPR, while the mortality in the three groups of the 30-33℃ experiment was observed as low as 2.3 to 6.5 per cent. 2) In the 20-24℃ experiment, the rectal temperature of WL at one day old was observed to be significantly higher than those of BPR and BPR×WL. In the 30-33℃ experiment, however, there was no significant difference in the rectal temperature of the three groups in each experiment. 3) The chicks survived at the low ambient temperature (20-24℃), though the growth rate was significantly lower than that of the 30-33℃ experiment, showed a significantly faster development (P<0.01) of homeothermic ability at two weeks of age. 4) The differences in the average release rates of thyroidal I^131 between the 20-24℃ and 30-33℃ experiments for each group were statistically not significant, whereas at 30-33℃ ambient temperature, the BPR showed higher thyroidal release rate of I131 (P<0.05) than either of the WL or the BPR×WL. The thyroid weight of chicks grown at the 20-24℃ ambient temperature was heavier (P<0.01) than that of the 30-33℃ experiment only in the BPR, while there was no significant difference in the WL and the BPR×WL.