Boar sperm oxidatively metabolized fructose, glucose, glycerol, glycerol 3-phosphate and lactate to CO2 but pyruvate produced only small amounts of CO2 and this was almost completely prevented when endogenous glycolytic metabolism was inhibited. Lactate was the preferred substrate over fructose, glycerol and glycerol 3-phosphate and when lactate was offered in the presence of pyruvate, lactate was preferentially oxidized to CO2. The rate of oxidation of fructose, glycerol and glycerol 3-phosphate was approximately halved in the presence of equi-molar concentrations of lactate and the metabolism of lactate was progressively decreased in the presence of increasing concentrations of mersalyl, an inhibitor of lactate transport. Sperm maintained a high energy charge potential when incubated with lactate as substrate in the presence or absence of bromopyruvate, an inhibitor of endogenous glycolytic metabolism. This evidence confirms that it is lactate, rather than pyruvate, that enters the mitochondria thereby constituting a lactate-pyruvate transport system in these cells for regenerating cytoplasmic nicotinamide adenine dinucleotide (NAD+). Electrophoretic examination of the lactate dehydrogenase isozymes from sperm and several other tissues of the boar showed that sperm contained almost entirely an isozyme which was not present in the other tissues.