Field studies of the gas exchange of Riesling grape vines (Vitis vinifera L.) showed large diurnal changes in net photosynthesis during the period of rapid sugar accumulation by fruit. The onset of the decline in photosynthesis occurred earlier in the day for vines without fruit. Decreases in photosynthesis were approximately proportional to changes in stomatal conductance. Despite the accompanying large changes in stomatal conductance, changes in intercellular CO2 concentration were small and mathematical analysis showed that stomatal changes accounted for only 20 to 40% of the change in assimilation rate. The CO2 compensation point, oxygen enhancement of photosynthesis, light and temperature responses of the leaf did not vary throughout the day. Stimulation of photorespiration was therefore not responsible for the non-stomatal inhibition of photosynthesis. No obvious correlation existed between the decline in leaf photosynthesis and daily changes in leaf water potential. A field study of Colombard grape vines revealed that frequently watered vines which had received irrigation 2 d prior to measurement showed little change in photosynthetic rate over the day compared with vines subjected to a more usual watering cycle, which had last been irrigated 18 d prior to measurement. Possible non-stomatal factors responsible for the reduction in photosynthesis during the day are discussed.