Previous work in many laboratories has shown that luteinizing hormone-releasing hormone (LHRH) neurons display a range of shapes from smooth-contoured to extremely irregular or 'thorny'. To determine if these forms reflect or can be modified by the gonadal steroid milieu, the morphology of LHRH neurons was examined in male rats from each of four experimental groups: adult (2-3 months); aged (23 months) and adult castrate (2-3 months, castrated for 4 weeks) with or without testosterone replacement. The region studied extended from the level of the medial septum and diagonal band of Broca posteriorly through the lateral preoptic and anterior hypothalamic areas, throughout which region the majority of LHRH neurons reside. Neurons were divided into groups based on the degree of irregularity of their profiles ('thorniness'). The total numbers of neurons did not differ among the experimental groups and smooth-countoured neurons predominated. However, there were fewer thorny LHRH neurons in the castrate animals than in the other experimental groups. LHRH neurons in the young adult and aged intact rats and testosterone-replaced animals did not differ in their morphology. These results suggest that LHRH neurons in the male rat tend to lose somatic or dendritic spines under conditions in which gonadal steroids are dramatically reduced. It also appears that the levels to which testosterone is reduced in the aged rat are not associated with a decrease in the incidence of spine-like projections on LHRH neurons.