The activities of the mevalonate metabolizing enzymes-HMG-CoA reductase, mevalonate kinase, mevalonate phosphokinase and mevalonate pyrophosphate decarboxylase -were assayed with the respective substrates in green seedlings of Arachis hypogaea. MVAPP decarboxylase is the rate-limiting step among these enzymes and is inhibited by phenolic acids. Its activity in the seedlings was found to decrease in the absence of light and on treatment with abscisic acid. These results suggest that regulation of isoprene pathway in groundnut seedlings may occur at the level of mevalonate decarboxylation.