Summary. Fluid was collected from the cannulated rete testis of six conscious Merino rams whose testes were heated to 40\m=.\5\s=deg\C for 3 hr. Rate of fluid flow dropped during heating but recovered thereafter; there were 100- to 10,000-fold decreases in sperm concentration which appeared to be biphasic. The timing supported the suggestion that heating had affected pachytene spermatocytes and division of type-B spermatogonia. The concentration of sodium, potassium, lactic acid, chloride, protein and testosterone in the fluid was unrelated to the number of spermatozoa present but above 20\m=x\106 spermatozoa/ml the concentration of inositol and glutamic acid appeared to be positively correlated with sperm number. Below this sperm concentration, small amounts of glucose appeared in some samples of fluid. The results clearly indicate that the secretion of rete testis fluid can be independent ofthe release of spermatozoa from the germinal epithelium.