13-Hydroperoxylinoleic acid (13-HPLA) inhibited glyoxalase II in rat liver supernatants with half the maximum inhibition occurring at a 13-HPLA concentration below 0.1 microgram/ml. The extent of inhibition was increased by EDTA and decreased by Mn2+ and Co2+. Zn2+ and Cu2+ inhibited control enzyme activity. Glyoxalase II catalyzes the destruction of S-lactoylglutathione. Both 13-HPLA and S-lactoylglutathione have been shown to potentiate histamine secretion. It is suggested that 13-HPLA does so, in part at least, by inhibiting glyoxalase II and thereby causing a concomitant increase in S-lactoylglutathione level. It is also possible that S-lactoylglutathione mediates other of the effects of 13-HPLA and fatty acid hydroperoxides in general.