Leaf isoprene emission rates (F iso) were studied in 2-year old trees of live oak (Quercus virginiana Mill.) during two drying-rewatering cycles. During the first drying-rewatering cycle, photosynthesis (A) and stomatal conductance (g s) decreased by 92%(±7%) and 91%(±8%), respectively, while F iso remained essentially constant for 8 days of treatment. After 12 days under severe drought conditions, F iso was reduced by 64%(±6%). Similar values were found during the second drying-rewatering cycle. During the recovery phase of both cycles, F iso recovered more quickly than A and g s. The lower drought sensitivity of F iso compared with that of A resulted in a higher percentage of fixed C lost as isoprene (C iso/C A) as plants became more stressed, reaching peaks of 50% when A was almost zero. F iso showed a strong negative linear relationship with pre-dawn leaf water potential (ψ PD) that could be a useful parameter to include in isoprene emission models to account for effects of drought stress on leaf F iso. © 2004 Elsevier Ltd. All rights reserved.