The active transport of sodium and potassium has been demonstrated in dog spermatozoa (Quinn & White, 1967) and the existence of a cation pump, involving a sodium—potassium activated ATPase has been postulated to account for the gradients of alkali metal ions between the cell and seminal plasma (Quinn, White & Wirrick, 1965). Recently, Uesugi & Yamazoe (1966) have reported the occurrence of a sodium—potassium activated ATPase in boar epididymal spermatozoa, and the present experiments were undertaken to determine the activity and distribution of ATPases in ram and bull spermatozoa. Semen was diluted with 2·5 vol of 250 mm-sucrose, centrifuged, and the spermatozoa washed twice with 5-ml aliquots of sucrose. Washed spermatozoa, resuspended in sucrose buffered to pH 7·4 (37° C) with 50 mm-tris (tris [hydroxylmethyl] amino methane) and 40 mm