Net photosynthesis (Pn) was determined on individual leaves of greenhouse grown ‘Golden Delicious’ on seedling rootstocks. Saturation light intensity varied among experiments, but was typically between 2000 and 4000 ft-c. Pn increased with increasing air flow rates up to about 2.5 liters min−l with only slight increases above this rate. Good comparisons of Pn could be made on either a dry wt or unit area basis within a similar set of trees. However, in comparing different groups of trees, the unit area basis gave a more suitable basis of comparison. No major diurnal trends were found nor did leaf size affect the Pn obtained. The data therefore, indicated that dependable determinations can be made at 4000 ft-c. and an air flow rate of 3 liters min−1.