The metabolism of boar semen differs in several respects from that of bull and ram. Anaerobically, boar spermatozoa convert fructose to lactic acid at a much lower rate than ram or bull spermatozoa, and under these conditions their motility is also characteristically small. Aerobically, boar spermatozoa convert fructose to lactic acid at the same or even lower rate, yet their motility is very high. This is due to the ability of boar spermatozoa to oxidize lactic acid with an efficiency as high as that encountered in ram or bull semen. Apart from lactic acid, boar spermatozoa are capable of utilizing aerobically a number of other substrates, including glycerol, pyruvic acid and acetic acid, but unlike ram spermatozoa, they lack the ability to oxidize sorbitol. Two other unusual features of the aerobic metabolism of boar semen are (i) an increase in oxygen uptake, which occurs after the semen has been stored in vitro, and (ii) the occurrence in the boar seminal plasma of an easily oxidizable substance which is responsible for the plasma's own oxygen uptake.