Simultaneous expression plasmids were constructed for bovine adrenal cytochromes P450cl7 and P450c21 (pAγα) and for both P450s together with NADPH-cytochrome P450 reductase (pARγα). On introduction of each of the plasmids into Saccharomyces cerevisiae AH22 cells, the transformed yeast strains AH22/pAγα and AH22/pARγα produced about 105molecules per cell of P450cl7 and 2 x 103molecules per cell of P450c21. The expression levels of NADPH-cytochrome P450 reductase was about 3 X 104and 6 X 105molecules per cell in the strains AH22/pAγα and AH22/pARγα, respectively. When progesterone was added to growing cell cultures of the transformed yeast strains, the substrate was metabolized more rapidly in the AH22/pARγα cells than AH22/pAγα cells, probably due to overproduction of the reductase. In the AH22/pARγα cells, progesterone was first converted into 17a-hydroxyprogesterone to the extent of 82% by the catalysis of P450cl7. 17α-hydroxyprogesterone was further converted into 11-deoxycortisol by P450c21 to the extent of 60% of the added substrate. The conversion of progesterone into androstenedione through 17α-hydroxyprogesterone was estimated to be less than 3% suggesting very low C17.20-lyase activity of P450cl7, although other hydroxylation products were detected. Androstenedione was further converted into testosterone by an unknown pathway present in S. cerevisiae cells.