Fumigation with isoprene and monoterpenes increases thermotolerance of isoprene-emitting and monoterpene-emitting species, respectively (1, 2). Isoprenoids are thought to stabilize membranes. Isoprene, however, does not protect against heat stress leaves which do not emit endogenous isoprene (1). This suggests that exogenous isoprenoids are ineffective when endogenous synthesis is absent. However, it may be possible that resistances to isoprenoid entry make their concentration in the leaf too low to be effective. A new technique has been recently shown to accurately mea:ure the concentration of isoprenoids in the leaves (3). We used this technique to measure the concentration of monoterpenes in the leaves of a non-emitting oak after fumigation with different dose F of monoterpenes. We collected new evidences that monoterpenes may increase thermotolerance also in non-emitting species, but the effect is noticeable more often at high doses of monoterpenes and on plants repeatedly exposed to heat stress.