We studied monoterpene emissions from the evergreen oak Quercus ilex exposed to different levels of summer drought to examine the seasonal variation of emissions in the Mediterranean area and to test the role of water limitations therein. Measurements were made in seven campaigns between June and January on intact leaves of mature trees growing in two adjacent sites, in one of which the natural water supply was reduced by a ground roof. In both sites, actual emission rates as well as light- and temperature-normalized emission rates (i.e., emission factor (EF)) significantly changed during the seasons: Mean EFs increased from June to July to a broad summer maximum between 5.2 and 9.4 nmol m-2 leaf area s-1 (12--21 ¿g g-1 leaf dry mass h-1), dropped in October and November and reached a minimum of about 0.77 nmol m-2 s-1 (1.7 ¿g g-1 h-1) in January. From June to July, mean EFs of the trees with reduced water supply were significantly lower than those of trees with normal water supply. The lowered EF in summer was paralleled by lower predawn water potentials, leaf transpiration, photosynthesis and stomatal conductance with respect to control trees. The results suggest that in natural Q. ilex habitats the seasonal evolution of EF follows a marked summer-winter cycle whose shape and intensity can be modified by summer drought.