Monospecific polyclonal antibodies to five constitutive hepatic microsomal cytochromes P-450 were prepared. These antibodies were used to monitor alterations in the content of the enzymes in livers of diabetic male rats. Within 3 wk of onset of streptozocin-induced diabetes, immunodetectable levels of RLM3 and RLM5 were decreased by 85 and greater than 95%, respectively. Insulin treatment for 1 wk reversed the decline in these isozymes and restored RLM3 to 60% and RLM5 to 53% of levels found in the untreated rat. After a 2nd wk of therapy, these levels were returned to 86 and 92%, respectively. In contrast, the levels of RLM5b and RLM6 were elevated in diabetes 1.7- and 8-fold, respectively. Insulin treatment for 1 wk only slightly decreased the levels of RLM5b but completely reduced RLM6 levels to those seen in age-matched untreated rats. After the 2nd wk of insulin treatment, the level of RLM5b was almost completely restored to normal, with no additional change in the RLM6 level. The level of a fifth enzyme, RLM5a, was not markedly altered by diabetes or by insulin treatment. The results suggest there are at least three types of responses by constituents of the cytochrome P-450 population to diabetes: no change in the microsomal content, a rapid increase when insulin level declines and restoration when insulin is supplied, and a rapid decline when insulin level declines and a restoration by insulin treatment.