OBJECTIVES: To study the role of opioids in early postnatal changes of the hypothalamic testosterone metabolism and catecholamine content underlying feminizing effect of prenatal stress on male sexual behavior in rats. MATERIAL AND METHODS: 10 day-old male and female offspring from mothers given naltrexone prior to daily 1-hour restraint during the last gestational week were used in the study. Aromatase and 5 alpha-reductase activities, noradrenaline and dopamine contents in the brain preoptic area and medial basal hypothalamus were studied by thin layer chromatography, radiometric and spectrofluorimetric techniques. RESULTS: Sexual dimorphism of testosterone metabolism enzymes activity and catecholamine content in discrete brain regions of 10 day-old rat pups was found. Prenatal stress attenuated these gender-related differences. Naltrexone pre-treatment of stressed dams prevented modifying effect of prenatal stress on aromatase activity and noradrenaline content in the male preoptic area. CONCLUSIONS: The results of the study demonstrate preventive effect of naltrexone on stress-induced alterations of testosterone aromatization and noradrenaline concentration in the developing brain preoptic area associated with neuroendocrine control of the male sexual behavior in adult rats. These findings indicate that endogenic opioids mediate detrimental effect of prenatal stress on neurochemical determinants of the brain sexual differentiation that may underlie feminization of the male sexual behavior.