Ejaculated spermatozoa were centrifuged from semen and their metabolism studied in media of various compositions. The optimum pH values for the endogenous respiration of bull, cock and rabbit spermatozoa were 6.9 to 7.0, 7.25 and 6.8, respectively. At the optimum pH for the sp. the endogenous respiration (measurement begun within 2 hrs. of ejaculation) of bull, cock, rabbit and ram spermatozoa were: Zo2 [long dash]21; 7; 11 and 22, respectively. Varying the phosphate conc. of the medium from 0 to 0.03 M did not greatly affect respiration of bovine spermatozoa, but glycolysis and motility in the presence of glucose were greatly depressed in the absence of phosphate. Mg++ and K+ were essential for maintenance of optimum motility, respiration and glycolysis. Mn++ and especially Ca++ affected motility, glycolysis, and respiration adversely.