STAR facilitates the fast increase in pregnenolone synthesis stimulated by tropic hormones. STAR increases pregnenolone synthesis more than four-fold and a major STAR transcription of 1.6 kb is observed in ovary and testis. Throughout the growth and differentiation period of the ovary follicle, the immunoreactivity is likely to move from the granulosa cells of early antral follicles to the theca cell layers in the adult.