The effect of fruiting on carbon fixation and retention in leaves was monitored by measuring net photosynthesis (Pn) and total non‐structural carbohydrates (TNC) on a seasonal basis on mature fruiting and non‐fruiting sweet cherry trees (Prunus aviumL. cv. Bing). Pn was also measured diurnally during stages II and III of fruit development. Pn rates increased to between 18 and 20 mg CO2dm‐2h‐1during stage II of fruit development and were maintained until harvest. Diurnally, Pn increased in the morning to 20 mg CO2dm‐2h‐1and this rate continued until sunset. Leaf carbohydrate levels decreased in both fruiting and non‐fruiting trees beginning at the equivalent of stage II of fruit growth. Carbohydrates were lower in leaves and woody portions of current, 1‐ and 2‐year‐old shoots of fruiting trees. Although differences were found in levels of non‐structural carbohydrates, no differences in Pn were found in fruiting vs non‐fruiting plants on either a seasonal or a diurnal basis. Pn rates in swet cherry in the field were primarily affected by ontogeny and environment a