We have previously shown that rat hepatic cytochromes P450f, P450g, P450h, and P450i possess a high degree of immunochemical and, presumably, structural relatedness. Polyclonal antibodies directed against cytochromes P450f and P450g were made monospecific by immunoabsorption against the cross-reactive proteins. The specificity of the immunoabsorbed antibodies was established by using Ouchterlony double diffusion analyses, enzyme-linked immunosorbent assays (ELISA), and immunoblots. Since factors regulating the expression of cytochromes P450f and P450g are unknown, a competitive ELISA employing the monospecific antibodies was developed to quantitate each of these isozymes in hepatic microsomes from control and treated rats. The results obtained showed that expression of cytochrome P450f is developmentally regulated in both male and female rat liver. Cytochrome P450f levels rise from less than 1% in young animals to approximately 7 and 14% of total cytochrome P450 in adult male and female rats, respectively. Cytochrome P450g is sex-specific since it is expressed only in male rat liver where it also is developmentally regulated. Levels of cytochrome P450g rise from less than 1% in 3-week-old male rats to an average value of 17% of total cytochrome P450 in 6-week-old adult animals. However, there appear to be at least two subpopulations of adult male Long Evans rats, one of which expresses low levels (less than 1%) of cytochrome P450g and the other high levels (greater than or equal to 10%). This expression appears to be independent of serum testosterone levels. Treatment of immature and adult male rats with 20 xenobiotics that are known inducers of certain cytochrome P450 isozymes revealed that cytochromes P450f and P450g are relatively refractory to induction, although Kepone appears to be a weak inducer of cytochrome P450f.