A striking sex-related difference in postpubertal growth and growth hormone (GH) secretory pattern in the rat has been described. Although this sexual dimorphism seems to be determined by the neonatal effects of gonadal steroids on the hypothalamus, peripubertal exposure to steroids also plays an important role. In order to study the real influence of the hypothalamic sex and/or peripubertal gonadal steroids, the growth pattern of female and male rats in response to neonatal and peripubertal sexual steroid treatments was studied using microknemometry, a technique that allows non-invasive daily measurements of rat tibial growth rate. Neonatal steroid environment in males was modified by castration on day 1, whereas in females it was changed by a single neonatal testosterone administration on day 5 followed by castration at 13 days of age. From the onset of puberty to adulthood, both female and male animals received testosterone or estrogens, respectively. Neonatal treatment alone, i.e. androgenization of female and castration of male rats, were only able to induce a partial reversal of the original sex-dependent growth pattern. Additional peripubertal treatments achieved a complete change in the sex-linked growth pattern. Consistent with the effects observed on growth, the pituitary GH concentration was significantly increased in females, and diminished in males, when they were treated both at the neonatal and peripubertal stages. However, only this latter group, whose growth was more seriously compromised, showed decreased plasma insulin-like growth factor-I (IGF-I) levels. In conclusion, a complete feminization of male tibial growth pattern or masculinization of female pattern can only be achieved by maintaining the new steroid environment from puberty to adulthood.