Functional differences exist between the porcine gonadal and placental aromatase cytochrome P450 (P450arom) isozymes that are encoded by separate genes. The present experiments investigated the sub-cellular location of these isozymes, their dependence on the redox partner protein NADPH-cytochrome P450 reductase (P450 reductase) for catalytic activity and the release of steroid intermediates during the aromatization of androgens to estrogen. After differential centrifugation, similar levels of gonadal and placental porcine P450arom were found along with P450 reductase in the microsomal compartment using activity and immunoblot analyses. Activity was stimulated much more by recombinant P450 reductase addition, and higher levels of 19-hydroxy and 19-oxo intermediates accumulated during androstenedione and testosterone metabolism, in cells expressing the gonadal compared to the placental isozyme. No other steroid products were identified by HPLC. Thus, the porcine gonadal P450arom is more sensitive to P450 reductase deprivation than is the placental P450arom, accounting in part for catalytic differences between these isozymes.