It has been reported earlier that nucleotides, nucleosides and a series of structurally related compounds as well as compounds based on transition state analogy inhibit yeast glyoxalase I. In our study on the metabolic regulation of glyoxalase I, we have found that nucleotides such as ATP, GTP and different classes of other reagents based on transition state analogy (D-isoascorbate, dihydroxyfumaric acid, rhodizonic acid) do not inhibit yeast or goat liver glyoxalase I. The reported inhibition of glyoxalase I by these compounds has been found to be due to the interference of these compounds with the absorbancy at 240 nm of S-D-lactoylglutathione formed by the glyoxalase I reaction. Glyoxalase I from goat liver has been found to be strongly and competitively inhibited by lactaldehyde. But, lactaldehyde has very little inhibitory effect on yeast glyoxalase I. Lactaldehyde is formed from methylglyoxal, the substrate for glyoxalase I by the enzyme methylglyoxal reductase. D-Lactaldehyde inhibits the liver enzyme more strongly than L-lactaldehyde. © 1987 Indian Academy of Sciences.