Diurnal and seasonal net photosynthetic rates (Pn) of sour cherry were determined. Leaf Pn was not significantly affected by shoot excision. Under constant environmental conditions (PFD, 1200 μmol m(-2)s(-1); temp. 25; relative humidity, 80-90%) there was no significant diurnal fluctuation in Pn for individual leaves. However, there was a pronounced fluctuation in Pn for whole trees measured under constant temperature but natural variation in sunlight from sunrise to sunset. Maximum Pn occurred before solar noon, remained constant for 1-2 hr, then declined. Photosynthetic rate of recently expanded leaves fluctuated through out the season but, in general, was greatest in the spring as leaves expanded, reached a peak, remained stable for several weeks, then gradually declined. The Pn of leaves on terminal shoots was not significantly different from the Pn of leaves on spurs of the same physiological age. The presence of fruit did not have a consistent effect on the Pn of sour cherry leaves.