Neonatal gonadectomy studies and hormonal replacement regimens were employed to characterize the regulation of delta 4-steroid 5 alpha-reductase, microsomal flavin-containing monooxygenase, and several forms of rat hepatic microsomal cytochrome P-450, including three that are sexually differentiated. Rats of both sexes that had been gonadectomized at birth were either untreated or were administered testosterone propionate or estradiol benzoate neonatally (subcutaneous injection on days 1 and 3 of life), postpubertally (an implant of a hormone-packed capsule at 5 weeks of age), or both neonatally and postpubertally. At the age of 10 weeks, all rats were killed, and several liver microsomal enzymes were assayed using immunochemical and catalytic techniques. Expression in the 10-week-old male and female rats of two male-specific cytochrome P-450 forms, termed P-4502c/UT-A and P-4502a/PCN-E, and their associated respective 16 alpha- and 6 beta-steroid hydroxylase activities could either be imprinted (programmed) by androgen exposure during the early neonatal period or, alternatively, could be stimulated by continuous hormone treatment after the age of 5 weeks. By contrast, hepatic expression of two female-specific enzymes, P-4502d/UT-1 and delta 4-steroid 5 alpha-reductase, was only partially dependent on estradiol; birth-gonadectomized rats expressed as much as 30-50% of the enzyme levels present in untreated adult females. Expression of both female-specific enzymes was fully suppressed upon postpubertal exposure to testosterone. In another study, birth sham-operated female rats were administered testosterone using the same regimens described above for the birth-gonadectomized rats. Although neonatal testosterone treatment alone did not affect the expression in these females of the four sex-specific enzymes examined in this study, it did enhance significantly the masculinization effected by postpubertal androgen exposure. This resulted in expression of the male-specific enzymes P-4502c/UT-A and P-4502a/PCN-E in these females at levels comparable to those found in adult males, while simultaneously suppressing the two female-specific enzymes, P-4502d/UT-I and delta 4-steroid 5 alpha-reductase, by approximately 70-75% to levels characteristic of prepubertal rats of either sex. The levels of another microsomal enzyme, flavin-containing monooxygenase, were also measured and found to be regulated by testosterone, but the ontogenic profiles and the effects of gonadectomy and hormone replacement indicated clear differences in its regulation when compared to the other male-specific enzymes.(ABSTRACT TRUNCATED AT 400 WORDS)