The effect of environmental temperature on the content of cytochrome p-450 or the activity of nicotinamide adenine dinucleotide phosphate-cytochrome c (NADPH-cytochrome c) reductase was examined by using microsomes prepared form rats kept at 30°C or 15°C for 14 d. The warm exposure (30°C for 14 d) resulted in a significant reduction of the activity of NADPH-cytochrome c reductase though the content of cytochrome P-450 was not changed. The level of thyroxine and 3, 3', 5-truodothyronine was reduced by the warm exposure. From the present observation, it was suggested that the effect of environmental temperature on the metabolism of drgus was due to the activity of NADPH-cytochrome c reductase mediated by the level of thyroid hormone.