Some of the contradictory postulates regarding gonadotropin action in infancy were reexamined. For the deprivation of newborn animals of endogenous gonadotropins, antiserum neutralizing theese hormones was administered from birth on. Using this approach it was elucidated that, even during the first week of life, gonadotropins stimulate development of ovarian somatic elements and are indispensable for this process. In the absence of the stimulatory action of endogenous gonadotropins, the normal ovarian development taking place during the first and second week of life was markedly altered and retarded. This was mainly evident from the reduction in the number of follicles which developed, the change in the appearance and ultrastructure of granulosa cells, the deficiency of thecal and of vascular development. That the observed alterations were due specifically to gonadotropin deprivation was proven by the fact that these alterations were prevented by the administration of human gonadotropins to mice deprived of endogenous gonadotropins. Substitution with human gonadotropins permitted the elucidation of the respective role of follicle-stimulating hormone (FSH) and FSH plus luteinizing hormone (LH) in the development of the infantile ovary. This was achieved by the use of HMG and of a biologically ‘pure FSH’ which was obtained by the neutralization of the LH activity present in HMG. FSH was found to be primarily responsible for the stimulation of granulosa cell proliferation, organization and structure. FSH plus LH, in addition to the FSH effects, initiated secretory activity of granulosa cells, increase in intrafollicular spaces, antrum formation, enrichment and maintenance of the theca layer and development of the vascular system. At the age of 14 days, the administration of HMG to gonadotropin-deprived animals even enhanced some of the developmental phases beyond that observed in ovaries of control animals at the same age. Oocyte growth seems to be independent of gonadotropins since it was similar in all experimental groups. The functional integrity of the growing oocytes in gonadotropin-deprived animals could not be assessed in this study.