The concentration of ferredoxin reductase, a component of the mitochondrial steroidogenic electron transport chain, was measured in the ovaries of pigs and superovulated rats by a protein blotting procedure using polyclonal antibodies to the purified protein. The concentration of ferredoxin reductase in porcine granulosa cells doubled during growth of follicles from small (1-2 mm diameter) to large (6-12 mm diameter). The concentration doubled again during the period of luteinization. This is in contrast to the rate of cholesterol side-chain cleavage, which showed little change during follicular growth but increased by more than tenfold during luteinization. A similar large increase in cholesterol side-chain cleavage occurs during the period of luteinization in the ovaries of superovulated rats, but as for the pig, only a small increase in ferredoxin reductase was observed. A threefold increase in the yield of mitochondrial protein from tissue homogenates was found between the granulosa cells of small-medium follicles and the cells of the corpora luteum. The increase in ferredoxin reductase during follicular development and luteinization, therefore, correlates well with the increase in mitochondria in the cells, but does not correlate with the dramatic increase in cholesterol side-chain cleavage activity which occurs during luteinization. Based on these results, it is unlikely that the level of ferredoxin reductase limits the expression of the full steroidogenic activity of the granulosa cells of the ovary.