The contraceptive steroid norethindrone caused a rapid time and dose-dependent loss of cytochrome P-450 from rat hepatocytes in suspension cultures. Up to 30% of this cytochrome was lost in the first 5 min of incubation; longer incubations resulted in little further loss even though not all the steroid was metabolised and the cells remained viable. Such cultures were used to investigate the formation of N-alkylated porphyrins (green pigments) which could be extracted from cell incubation mixtures following exposure to norethindrone and separation by HPLC or TLC. The number of N-alkylated porphyrins formed was dependent both on the time of incubation and the concentration of steroid. After 1 min, 1 major green pigment (GP1) was resolved using either high (0.3 mM) or low (0.03 mM) norethindrone concentrations. With longer incubation times (60 min), at high steroid concentrations, only one additional polar adduct (GP2) was formed. At lower steroid levels, 3 more polar components (GP2, 3 and 4) were seen. As judged by HPLC or TLC, GP1 corresponds to the pigment formed in microsomal preparations incubated with norethindrone in vitro, while GP2, 3 and 4 correspond to the pigments extracted from the livers of rats administered this steroid in vivo. Pretreatment of rats with either phenobarbitone or 3-methylcholanthrene induced cytochrome P-450s. Relative to controls, phenobarbitone pretreatment also resulted in a greater accumulation of green pigments in hepatocytes incubated with norethindrone, the more polar forms of green pigments (GP3 and 4), showing a disproportionate increase in concentration. The mixed function oxidase inhibitor SKF 525-A or high concentrations of steroid not containing an ethynyl function, e.g. norethandrolone, when added to cell cultures containing norethindrone, preferentially inhibited the formation of GP3 and 4. When purified green pigments were added to cell incubation mixtures in the absence of norethindrone, preferentially inhibited the formation of GP3 and 4. When purified green pigments were added to cell incubation mixtures in the absence of norethindrone, no interconversion of one form to another could be demonstrated. The results suggest that the more polar norethindrone-protoporphyrin IX adducts (GP2, 3 and 4) arise as a result of metabolic modification of norethindrone rather than the protoporphyrin IX moiety, either prior to or after activation of the ethynyl function. The formation of several green pigment components in hepatocyte suspensions was not unique to norethindrone but occurred with a number of other 17-ethynyl-substituted steroids.