The cellular localization of substance P immunoreactivity was demonstrated at light microscopy level in hamster gonads during foetal and postnatal development. Selective immunostaining was observed in both the foetal and the adult generation of Leydig cells, but only in the luteal and interstitial cells of the adult ovary. To date there have been few studies dealing with the potential role of substance P in the control of gonadal functions. To examine whether it exerts an effect on gonadal steroidogenesis, the gonads of newborn, prepuberal and adult hamsters were cultured in vitro in the presence of substance P (10(-7) M), HCG (5 IU/ml) or substance P + HCG. Production of the sex steroid hormones testosterone, oestradiol-17 beta and progesterone was measured by radioimmunoassay. Substance P was observed to have a modulatory effect on steroid production by foetal, immature and mature gonads of both sexes.