Little work has been done on the biosynthesis of plant terpenes, and in order to investigate the experimental techniques of incorporation we have fed Msf4C02E into citronellal in Rucalsptus citriodora Rook and 2-14C-mevalonic lactone into oineole in &globulus.Lsb. Terminal branchlets (total weight about 50 g) were allowed to absorb by transpiration acqueous solutions (about 30 so) containing the labelled precursor, assisted if neceasaxy by a current of warm air. The oils mere obtained by steamdistillation (cohobation technique), citronella1 being Isolated as its 2rr(-dinitrophenylhydramone, and ulneole ae the reeorofnol oomplex. Dilution with inactive material (about 200 mg) assisted puriffoation and provided sufficient material for degrsdation. The inoorporatlons were small, but sufficient for further investigation. The pattern expeoted for citronellal ia (I) and with 14el%02R precursor the activity should be aero if no randomisation owurs. The result shown, based on Kuhn-Roth oxidation, aupports the expeated distribution although randomiestion ie oonsiderable, the aocuraoy befng good in view of rather low aotivities [&ound for the whole moleoulet r.m.a. 11 x 10 3 1 oalo. from the mm of the r.m.a. oontrlbutlons based on(I) and the results of the Kuhn-Roth oxidation: r.m.a. 10 x 10^3]. The randomisation may occur through formation of 14CO2 and photosynthesis. The pattern expected for cineole from 2-14C-mevalonic lactone is (II) and is supported by the degradation shown: little if any randomisation appears to have occurred. The degradation assumes that the isotope effect in the oxidation to cineolic acid is small. Clearly, this technique can be used for other leaf terpenes, especially with mevalonic lactone. While this work was in progress Stanley ¹ showed that mevalonic acid could be incorporated into a-pinene, but did not degrade the product.