We investigated the potential importance of the basal follicle-stimulating hormone (FSH) and luteinizing hormone (LH) release rates in causing the acute and chronic elevations in serum FSH and LH concentrations which occur after ovariectomy (OVX) and orchidectomy (ORCH) of rats. Diestrous day 1 female and male rats were decapitated or castrated and killed 2, 4 or 8 h or 1, 2, 7, 21 or 35 days later. In females, the weight of the anterior pituitary gland (APG) did not change. Serum FSH rose within 4 h and then progressively higher until 35 days after OVX. These increases were paralleled nearly perfectly with increases in APG FSH concentration which was first elevated at 1 day after OVX and in the basal FSH release rate (measured in vitro) which was first elevated at 4 h after OVX. Serum LH levels rose by 7 days after OVX and then more dramatically thereafter. These increases were associated with increased APG LH concentrations. The pronounced increases in serum LH levels between 7 and 35 days after OVX were associated with marked increases in the basal LH release rate. In males, APG weight was increased at 21 and 35 days after ORCH. Serum FSH levels were elevated at 1 day after ORCH and continued to rise until 21 days after ORCH. APG FSH concentration was decreased at 2 and 7 days and increased at 35 days after ORCH. The basal FSH release rate per milligram APG did not change significantly after ORCH. Serum LH levels were elevated at 8 h after ORCH. They rose further by 1 day and then further between 7 and 21 days after ORCH. APG LH concentration and the basal LH release rate per milligram APG were elevated at 21 and 35 days after ORCH. The results suggest that changes in basal FSH and LH release are (1) involved to a major extent in causing the post-OVX rise in serum FSH concentration during the first 5 weeks after OVX and in serum LH concentration between 7 and 35 days after OVX, (2) not involved in causing the post-ORCH rises in serum FSH and LH concentrations during the 1 week after ORCH, and (3) involved to some extent in causing the elevations in serum FSH and LH concentrations observed at 3 and 5 weeks after ORCH. The results also indicate that (1) increases in the basal gonadotropin release rates per milligram APG after castration may be coupled in some way with increased synthesis of gonadotropin, and (2) increases in the basal LH release rate per milligram APG can occur independently of an increase in the basal FSH release rate per milligram APG.