The growth hormone (GH) secretory pattern changes significantly throughout development in both male and female rats, becoming markedly sexually dimorphic after pubertal onset. This observation suggests that pubertal sex steroids play a role in the manifestation of this phenomenon. The neonatal steroid environment has also been shown to be intricately involved in the generation of the final adult GH secretory pattern, but the mechanisms underlying this effect remain unknown. We have addressed the question as to whether the developmental changes in the GH secretory pattern are correlated with changes in the hypothalamic neuropeptides that regulate its release from the anterior pituitary, i.e., somatostatin (SS) and growth hormone-releasing hormone (GHRH). The effects of neonatal testosterone and adult testosterone treatments on these two neuropeptide systems have also been studied. We have found that the synthetic capacity, as reflected in relative messenger RNA (mRNA) levels, of both SS and GHRH neurons changes throughout development in both male and female rats. These mRNA levels are also sexually dimorphic at certain times during maturation and, at least in the adult male, can be modulated by changes in testosterone levels. In support of the hypothesis that sex steroids play a role in the organization of the developing hypothalamus, we have shown that both estradiol and testosterone promote the survival of hypothalamic neurons in vitro. Preliminary in vivo studies indicate that the neonatal sex steroid environment may influence the number of GHRH neurons that are found in the adult brain, as well as their sensitivity to adult steroids.(ABSTRACT TRUNCATED AT 250 WORDS)