Isoprene (2-methyl-1,3-butadiene) is a major volatile hydrocarbon produced by many plant species. Here we report that in velvet bean (Mucuna sp.), isoprene emission is strongly dependent on leaf developmental state and that changes in extractable isoprene synthase activity parallel isoprene emission rates during leaf development. Both leaf emission and enzyme activity exhibit over 100-fold increases from leaf emergence to leaf age 14 d and exhibit similar patterns to 23 d. This suggests that the enzyme, isoprene synthase, is responsible for the in vivo production of isoprene and that the level of the enzyme regulates the pattern of isoprene emission in response to leaf development.