Ginsenoside-Rb1 is one of the pharmacologically active components of ginseng, the dry root of Panax ginseng C. A. Meyer (Araliaceae), a well-known traditional Chinese medicine. Ginseng enhanced mounting behaviour of male rats and increased sperm counts in rabbit testes. Some experimental results suggested no sex hormone-like function in ginseng but probably gonadotropin-like action. The present study was to explore the effect of ginsenoside-Rb1 on the secretion of luteinizing hormone (LH) both in vivo and in vitro. Male rats were orchidectomized (Orch) for 2 weeks or subjected to swim training for 1 week before catheterization via the right jugular vein. They were intravenously injected with ginsenoside-Rb1 (10 microg/kg) or saline at 15 min prior to a challenge of gonadotropin-releasing hormone (GnRH) or 10 min-swim. Blood samples were collected at several time intervals following intravenous injection of ginsenoside-Rb1. In the in vitro experiment, male rats were decapitated and their anterior pituitary gland (APs) were either bisceted or enzymatically dispersed. The hemi-APs were preincubated with Locke's medium at 37 degrees C for 90 min and then incubated with Locke's medium containing ginsenoside-Rb1 (10(-7) - 10(-4) M) for 30 min. The dispersed AP cells (1 x 1(5) cells per well) were primed with dihydrotestosterone (DHT, 10(-8) M) for 3 days, and then challenged with ginsenoside-Rbl (10(-6) and 10(-5) M, n = 8) for 3 h. The concentrations of LH or testosterone in samples were measured by radioimmunoassays. Administration of ginsenoside-Rb1 did not alter the levels of plasma LH in both intact and Orch rats but significantly increased plasma LH concentration at the termination of the 10 min swimming exercise. Administration of ginsenoside-Rb1 resulted in a lower testosterone response to GnRH challenge or swimming exercise as compared with saline-treated rats. Ginsenoside-Rb1 dose-dependently increased the release of LH from both hemi-AP tissues and the DHT-primed dispersed AP cells in vitro. These results suggest that ginsenoside-Rb1 increases LH secretion by acting directly on rat AP cells.