The metabolism of the progestogen oral contraceptive norgestimate has been studied in vitro using human intestinal mucosa and human liver microsomes. Metabolites have been separated using radiometric high-performance liquid chromatography (HPLC) and identified by co-chromatography with authentic standards and by mass spectrometry. Histologically normal colon was obtained from 6 patients undergoing various resections and the mucosa mounted between 2 perspex (Ussing) chambers. 2 h after addition of [3H]norgestimate to the mucosal chamber, more than 95% of the radioactivity was present in that chamber. Metabolite analysis showed 38.1 +/- 11.6% (mean +/- SD; n = 8) of drug present was norgestimate, 49.2 +/- 14.5% as 17-deacetyl norgestimate and 8.1 +/- 4.5% as conjugated metabolites. Small amounts of 3-keto norgestimate, norgestrel and uncharacterized metabolites were found. Norgestimate was also metabolized by stomach tissue with 17-deacetyl norgestimate again being the main metabolite found. Microsomes were prepared from 6 human livers. Metabolism was studied over a 5 h time-course in the absence and presence of NADPH. Deacetylation to 17-deacetyl norgestimate took place in the absence of the cofactor. In the presence of NADPH, after 5 h incubation only 30.5 +/- 14.6% (mean +/- SD) of steroid present was norgestimate. The major metabolite formed was 17-deacetyl norgestimate which accounted for 39.3 +/- 20.5%. Less than 2% was present as 3-keto norgestimate but 10.0 +/- 2.3% was identified as norgestrel and 15.5 +/- 8.9% as uncharacterized metabolites. We also examined the microsomal breakdown of [3H]17-deacetyl norgestimate. This was NADPH and oxygen dependent. Norgestrel and other metabolites were formed. This study has demonstrated that norgestimate is rapidly deacetylated by both gut wall and liver. The deacetylated metabolite can then be further metabolized.