Alterations in heme biosynthetic and degradative capabilities and in the activities of several heme-containing enzymes were examined in hepatic tissues of streptozotocin (STZ)-diabetic female Sprague-Dawley rats. Activities were measured 10, 30 and 90 days following the administration of STZ (65 mg/kg, i.v.). The activities of the key enzymes involved in heme synthesis, delta-aminolevulinic acid (ALA) synthase, ALA dehydratase, and uroporphyrinogen synthase, were decreased markedly in STZ-diabetic rats as compared to sham-operated animals. Furthermore, the catabolism of heme which occurs via microsomal heme oxygenase (MHO) remained unaltered in these animals. Microsomal content of heme and cytochrome P-450, and the activities of tryptophan pyrrolase and the drug-metabolizing enzymes benzo[a]pyrene (BP) hydroxylase and aniline hydroxylase, were increased in the livers of diabetic rats. By contrast, the activity of the heme-containing enzyme catalase was decreased in these animals. Cobalt chloride produced a marked increase in MHO with a concomitant decrease in microsomal content of cytochrome P-450 and its associated BP hydroxylase activity in normal as well as chronically diabetic rats. It was of interest, however, that the increase in ALA synthase that is normally produced by this metal was not seen in chronic diabetic animals. Thus, chronic diabetes produced subtle and important disruptions in cellular metabolism, which may have been the result of long-term alterations in key enzymes involved in heme synthesis.