The effects of glyoxylate plus oxaloacetate and of oxalomalate on the NADP-linked isocitrate dehydrogenase (threo-DS-isocitrate:NADP+ oxidoreductase (decarboxylating, EC 1.1.1.42) from pig heart from been studied with steady state methods as well as with stopped flow technique. When equimolar mixtures of glyoxylate and oxaloacetate were premixed for different lengths of time prior to addition to the assay mixture, the extent of inhibition increased with the premixing time. The results indicated that the inhibition by glyoxylate plus oxaloacetate is caused by a compound formed in a reversible interaction between the two components. Glyoxylate plus oxaloacetate and oxalomalate affected the enzyme in at least three different ways. They inhibited the enzyme in a reaction competitive with regard to the substrate isocitrate. This inhibition needed a certain time to be fully expressed. The time lag could be eliminated by premixing of the enzyme and inhibitor with NADP plus metal ion. Secondly, if the enzyme is premixed with NADP plus metal ions, a time lag occurs before the reaction rate approaches a constant value after initiation of the reaction with isocitrate. The inhibitors were found to enhance this effect of NADP plus metal ions on the enzyme. Thirdly, it has previously been shown that the enzyme can be activated by metal complexing agents. Glyoxylate plus oxaloacetate as well as oxalomalate are able to form complexes with metal ions and were found to cause an initial activation of the enzyme under certain assay conditions. The controversy regarding the mechanism of action of the above inhibitors on the enzyme is probably due to the fact that they affect the enzyme in several different ways.