We have examined the effect of dietary protein deficiency on rat hepatic drug-metabolizing enzyme system for a period of two months. Cytochrome P-450 and b5 contents in liver microsomes, which were plotted on semilogarithmic paper as a function of the time of deficiency, showed biphasical reductions during protein deficiency: rapid decreases in the first 3 weeks were followed by more gradual decreases. However, the three enzymatic activities examined, i.e. aminopyrine demethylase, aniline hydroxylase and p-nitroanisole demethylase, were not reduced at a uniform rate. In the earlier phase, activities of the former two enzymes were reduced more rapidly than that of the last phase. This biphasical and non-uniform reduction of enzymatic activities suggests the existence of two or more cytochrome P-450 subspecies in non-depleted male rats. Intraperitoneal administration of well-known environmental pollutants, polychlorinated dibenzofurans and biphenyls (100 micrograms and 100 mg/kg, respectively) to the depleted rats resulted in a marked induction of drug-metabolizing enzymes. However, as the deficiency became more severe (2 months), the induction declined to a considerable degree, especially in the case of polychlorinated biphenyl administration.