Integrity of the thymus during perinatal life is necessary for a proper maturation of the pituitary-gonadal axis in mice and other mammalian species. Thus, congenitally athymic (nude) female mice show significantly reduced levels of circulating gonadotropins, a fact that seems to be causally related to a number of reproductive derangements described in these mutants. Interestingly, a number of in vitro studies suggest that the thymic peptide thymulin may be involved in thymus-pituitary communication. In order to determine the consequences of low serum thymulin in otherwise normal animals, we induced short-(8 days) and long-(33 days) term thymulin deficiency in C57BL/6 mice by neonatally injecting (i.p.) an anti-thymulin serum and assessed their circulating gonadotropin levels at puberty and thereafter. Control mice received an irrelevant antiserum. Gonadotropins were measured by radioimmunoassay and thymulin by bioassay. Both long- and short-term serum thymulin immunoneutralization resulted in a significant reduction in the serum levels of gonadotropins at 33 and 45 days of age. Subsequently, we injected (i.m.) an adenoviral vector harboring a synthetic DNA sequence (5-ATGCAAGCCAAATCTCAAGGTGGATCCA-ACTAGTAG-3) encoding a biologically active analog of thymulin, metFTS, in newborn nude mice (which are thymulin deficient) and measuerd circulating gonadotropin levels when the animals reached 52 days of age. It was observed that neonatal thymulin gene therapy in the athymic mice restored their serum thymulin levels and prevented the reduction in circulating gonadotropin levels that typically emerges in these mutants after puberty. Our results indicate that thymulin plays a relevant physiologic role in the thymus-pituitary-gonadal axis.