Evidence from studies in vitro supports the concept that growth factors modulate endocrine-dependent differentiative processes in follicle development. Based upon results from granulosa cell culture systems, it is suggested that the study of growth factors and their regulatory mechanisms (endocrine, paracrine, autocrine control) could perhaps be generalized to other areas concerned with the regulation of steroid secretion such as placental physiology, the regulation of fetal gonads and puberty, secondary steroid metabolism and steroid-secreting tumors.