The effect of environmental temperature on cytochrome P-450 and the associated electron transfer system was studied in male rats. Rats kept at 15°C for two weeks (group A) showed a significant increase in the activity of nicotinamide adenine dinucleotide phosphate reduced form (NADPH)-cytochrome c (P-450) reductase, whereas they showed a slight reduction in the content of cytochrome P-450 when compared with rats kept at 30°C for two weeks (group B). Further, aniline hydroxylase activity in group A increased significantly, while aminopyrine N-demethylase activity was unchanged. The activity of NADH-cytochrome b₅ reductase and the content of cytochrome b₅ showed no significant difference between the two groups. From the endocrinological viewpoint. the serum level of 3, 5, 3'-triiodothyronine (T₃) in group A was higher than that in group B. The differences in enzyme activity and content were observed from 4d after the start of exposure of rats to each temperature.