1. The relative roles of pituitary hormones (especially growth hormone) and testicular hormones (especially testosterone) in the regulation of renal and hepatic CYP2E1 and cytochrome P450 reductase have been studied in the male rat. 2. Depletion of pituitary hormones by hypophysectomy (Hx) resulted in 12-14-fold increases in renal CYP2E1 (p < or = 0.05) and a 40% drop in NADPH-dependent cytochrome c reductase activity (p < or = 0.05) compared with 6-fold increases in CYP2E1 (p < or = 0.05) and a 60% drop in P450 reductase apoprotein (p < or = 0.05) in the liver. 3. The increase in hepatic CYP2E1 was associated with increased gene transcription in nuclear run-on experiments. 4. Restoration of renal CYP2E1 to control levels by hormone treatment required both growth hormone and an intact testis, whereas partial restoration of CYP2E1 apoprotein levels in liver was accomplished by growth hormone, but not testosterone. 5. Renal NADPH-dependent cytochrome c reductase activity was restored by growth hormone and testosterone treatment, whereas the hepatic reductase appeared to be regulated by other pituitary hormones. 6. CYP2E1 and P450 reductase appear to be under complex endocrine regulation by pituitary and testicular hormones in a tissue specific manner.