The influence of water stress on gas exchange and dry matter accumulation in a wild sunflower (Helianthus agrophyllus T&G) and four cultivated sunflower (Helianthus annuus L.) cultivars (HA 89, RHA 856, 207 A and C) was compared in a glasshouse experiment during 1989-1990. Plants were maintained in a well-watered condition until physiological maturity, while in the drought stress treatment, the irrigation was suspended just before flowering. Measurements were made of leaf photosynthetic and transpiration rates, relative leaf water content, pre-dawn leaf water potential, leaf area index and total dray matter at harvest. Under dray conditions, the water deficit was smaller in the wild species than in the cultivated lines, and photosynthesis and leaf hydration decreased more rapidly with drought in the cultivars. These results, together with the wild sunflower's smaller reduction in dry matter production, its increase in root/shoot ratio under drought at harvest and the unchanged leaf area after 12 days ofdrought, indicate it has a dehydration avoidance mechanism. The hydration avoidance may be due to the larger roots of H. argophyllus and higher root/shoot ratio under drought, compared with the cultivars.