Previous studies have shown sex differences in intramembrane particle content in the arcuate neurons of the rat hypothalamus. In this study, freeze-fracture replicas were prepared from the infundibular hypothalamus of adult African green monkeys (Cercopithecus aethiops) in order to determine whether primates also have sexual dimorphism in neuronal membranes. Intramembrane particles (IMP) were quantitatively assessed in the perikaryal plasma membranes of infundibular neurons. Four groups of monkeys were studied: intact males, intact females, ovariectomized females injected with 20 mg of estradiol valerate over 10 days and ovariectomized females injected with vehicle (castor oil). Membranes from females showed an increased numerical density of IMPs when compared to males. Ovariectomy of females did not affect IMP content, while estrogen administration resulted in a significant decrease in IMP numerical density to reach male values. These findings indicate a sex difference in neuronal membranes in the hypothalamus of monkeys and suggest that as in rodents, neuronal plasma membrane organization in higher primates may be modulated by gonadal steroids.