Northern analysis was used to identify relaxin gene expression in ovaries of prepubertal pigs primed with pregnant mare's serum gonadotrophin (PMSG) and human chorionic gonadotrophin (hCG). The cellular distribution of relaxin transcript in the developing follicle was localized by in-situ hybridization histochemistry. Three probes complementary to non-overlapping regions of the porcine prorelaxin molecule were used to identify relaxin gene expression in ovarian follicular tissue collected 0, 48, 60, 72 and 84 h after treatment with PMSG/hCG. A 1 kb transcript was detected in ovarian extracts of prepubertal gilts from 48 to 84 h after PMSG stimulation. This corresponds to the molecular size of the relaxin transcript reported in the pregnant sow ovary. Relaxin mRNA levels increased in ovaries from animals 48 through 84 h after PMSG. In-situ hybridization showed that the site of relaxin synthesis was the theca interna layer of the developing follicle. Relaxin mRNA was not observed in other follicular cell types, in small or atretic follicles or in follicles from unstimulated animals. The distribution and relative concentration of relaxin mRNA showed a good correlation with in-vitro production and immunohistochemical localization of relaxin previously reported in the developing pig follicle. The presence of both protein and mRNA for relaxin in the growing follicle supports a role for relaxin as a local regulator of ovarian function.