A possible physiologic role for the concerted inhibition of a NADP +-specific isocitrate dehydrogenase by oxalacetate and glyoxylate is presented. The significance of this inhibition is also discussed with respect to the “induced fit” theory of enzyme-substrate interaction. However, since numerous structural analogues of these two compounds were without effect, it is believed that these two components are not acting as a “synthetic” substrate.