A technique which depends on absorbing 14CO2 on paper strips containing small amounts of KOH, and 'counting' the strips directly in a liquid-scintillation system, has been applied to studies on spermatozoal metabolism. A small quantity of uniformly-labelled [14C]-glucose or [14C]-fructose has been added to washed spermatozoal suspensions which already contained non-labelled glucose or fructose and the 14CO2 production assessed at the end of a 1 hr incubation period in an air atmosphere, at 38° C. Boar spermatozoa, like those of ram, bull and man, have been shown to utilize glucose preferentially if presented with a mixture of these two sugars.