Glycerokinase occurs in ejaculated bull spermatozoa. Characteristics of the spermatozoan glycerokinase are quite similar to those of rat liver glycerokinase, except that the enzyme is firmly bound to the midpieces (mitochondria). The addition of l-glycerol-3-phosphate, as well as of free glycerol, to the washed bull spermatozoa causes both an increase in oxygen uptake and an accumulation of lactic acid. Since bull spermatozoa possess neither NAD+- nor NADP +-linked glycerol dehydrogenase, l-glycerol-3-phosphate would be formed by the action of glycerokinase as the initial step of glycerol metabolism in bull spermatozoa, followed by its oxidation to dihydroxyacetonephosphate by the mitochondrial dehydrogenase, via the glycolytic pathway. Among other animals, fowl and ram spermatozoa show high specific activity of glycerokinase. The enzyme activity is low in boar spermatozoa, and is not detectable in rabbit, human, rainbow trout, starfish and seaurchin spermatozoa.