A soluble enzyme system obtained from Pinus radiata seedlings incorporates 6–7% of radioactivity from DL-2- 14C mevalonic acid into isopentenol, dimethyl allyl alcohol, nerol, and geraniol. The system requires Mg 2+ or Mn 2+, and SH groups for optimal activity. The C 5, and C 10 prenols are formed from the correspondent phosphorylated compounds by a phosphatase, which may be partially inhibited by NaF. About 1% of the radioactivity from mevalonic acid is found loosely bound to proteins. It may be extracted with 1-butanol and appears associated with C 15 prenols. Previous chemical evidence and some data in this paper suggest that neryl pyrophosphate and not geranyl pyrophosphate may be the precursor of cyclic monoterpenes.