The triazole derivative, R 76713 and its enantiomers R 83839(-) and R 83842(+) are effective inhibitors of the aromatization of androstenedione. For human placental microsomes, the (+) enantiomer (R 83824) is about 1.9- and 32-times more active than the racemate (IC50 2.6 nM) and the (-) enantiomer, respectively. R 83842 is about 30- and 1029-times more active than 4-hydroxyandrostene-3,17-dione and aminoglutethimide. This potency might originate from its high affinity for the microsomal cytochrome P450 (P450). Indeed, R 83842, compared to R 76713 and R 83839, forms a more stable P450-drug complex. Difference spectral measurements indicate that the triazole nitrogen N-4 coordinates to the haem iron. The reversed type 1 spectral changes suggest that R 76713 is able to displace the substrate from its binding place and the stable complex formed in particular with the (+) enantiomer suggests that its N-1-substituent occupies a lipophilic region of the apoprotein moiety. Kinetic analysis implies that there is a competitive part in the inhibition of the human placental aromatase by R 76713. The Ki values for R 76713, R 83842 and R 83839 are 1.3 nM, 0.7 nM and 18 nM, respectively. These results are indicative of stereospecificity for binding. Up to 10 microM, R 76713 and its enantiomers have no statistically significant effect on the regio- and stereoselective oxidations of testosterone in male rat liver microsomes. All three compounds have no effect on the P450-dependent cholesterol synthesis, cholesterol side-chain cleavage and 7 alpha-hydroxylation and 21-hydroxylase. At 10 microM, R 76713 has a slight effect on the bovine adrenal 11 beta-hydroxylase. This effect originates mainly from R 83839, the less potent aromatase inhibitor. On the other hand, the inhibition of the 17,20-lyase of rat testis observed at concentrations greater than or equal to 0.5 microM, originates rather from R 83842. However, 50% inhibition is only achieved at 1.8 microM R 83842, i.e. at a concentration about 1300-times higher than that needed to reach 50% inhibition of the human placental aromatase.