(S)-α-Chlorohydrin inhibits the conversion of fructose to lactate by mature guinea pig spermatozoa in vitro . At a concentration of 2mM, there is a specific inhibition of glyceraldehyde-phosphate dehydrogenase resulting in the accumulation of fructose-1,6-bisphosphate, dihydroxyacetone phosphate and glyceraldehyde-3-phosphate and a concomitant decrease in the concentration of endogenous lactate. (R,S)-[3- 36Cl]-α-Chlorohydrin is metabolised by the spermatozoa to 3-chlorolactaldehyde of unknown configuration. Exogenous (R,S)-3-chlorolactaldehyde (2mM) is an inhibitor of glyceraldehyde-phosphate dehydrogenase in guinea pig spermatozoa, whereas (R)-α-chlorohydrin (10mM) has no significant effect on the metabolism of fructose. 6-Chloro-6-deoxyglucose (10mM) inhibits glyceraldehyde-phosphate dehydrogenase in guinea pig spermatozoa in vitro and is metabolised to 3-chlorolactaldehyde which is presumably the (S)-isomer. The anti-glycolytic actions of (S)-α-chlorohydrin and 6-chloro-6-deoxyglucose in guinea pig spermatozoa are suggested to be due to the action of a common metabolite, (S)-3-chlorolactaldehyde.