When boar spermatozoa were incubated with the (S)-isomer of the male antifertility agent a-chlorohydrin the activity of glyceraldehyde-3-phosphate dehydrogenase was inhibited. The (R)-isomer had no significant effect on the activity of this enzyme whereas (R,S)-3-chlorolactaldehyde caused an inhibition of its activity and also in that of lactate dehydrogenase. The in vitro production of (S)-3-chlorolactaldehyde, the active metabolite of (S)-a-chlorohydrin, was attempted by incubating boar spermatozoa with l-chloro-3-hydroxypropanone. Preliminary results lead us to propose that this compound is converted into (S)-3-chlorolactaldehyde as well as to another metabolite which is an inhibitor of other enzymes within the fructolytic pathway.