It has already been reported that rete testis fluid (RTF) from rams and boars contains something which reduces the uterine response to human chorionic gonadotrophin in rats and mice, presumably by reducing the secretion of endogenous gonadotrophin by the recipient animals (Setchell & Sirinathsinghji, 1972). These observations have now been extended by measurements of the concentrations of follicle-stimulating (FSH) and luteinizing hormones (LH) in the plasma of rats injected with RTF. Three groups of recipient rats were used: 21-day-old females, adult, intact males, weighing between 250 and 400 g, and adult males of similar age, castrated 3 weeks previously. Ram RTF was collected at 4 °C, and the spermatozoa were removed by centrifugation (3000 g for 15 min); the fluid was kept at −20°C until used. The fluid was injected intraperitoneally, twice daily, 0·5 ml at 09.00 h and 1·0 ml at 17.00 h; some of the fluid was concentrated