The relationship of opiate receptors in the medial preoptic area of the hypothalamus (MPOA) to the gonadal steroid hormone environment during development was assessed using regional densitometric analyses of [3H]naloxone binding in autoradiographs prepared using brain sections from 5-day-old male and female rats treated postnatally either with tamoxifen (0.5 mg/kg), flutamide (20 mg/kg), dihydrotestosterone (DHT; 12.5 mg/kg), or sesame oil vehicle. Tamoxifen, a specific estrogen receptor antagonist, did not alter MPOA binding density in either males or females. Flutamide, a specific androgen receptor antagonist, and DHT, a nonaromatizable androgenic compound, altered the MPOA binding density in males and females, respectively. No treatment altered the binding density in several other brain regions. The results suggest that androgen, not estrogen, regulates the differentiation of MPOA opiate receptors. Since the neuronal development in the region is thought to be mediated by estrogen, both hormones probably act concurrently to affect the ontogeny of different parameters in the same brain region.