The effect of clofibrate on the activity of the three mevalonate-activating enzymes has been studied for the first time in brain by reactions carried out using [2-14C] mevalonic acid as substrate and 105,000 g supernatants from 14-day-old chick brain. Mevalonate-5-pyrophosphate decarboxylase was clearly inhibited, while mevalonate kinase and mevalonate-5-phosphate kinase were not significantly affected. The effect of clofibrate on decarboxylase activity was progressive with increasing concentrations (1.25-5.00 mM) of the inhibitor. A transient inhibition and a subsequent activation as a function of clofibrate concentration seemed to occur for mevalonate kinase. Direct measurements of decarboxylase activity utilizing [2-14C] pyrophosphomevalonate as the specific substrate of this enzyme corroborated these results. Kinetic studies showed that clofibrate competes with the substrate ATP.