Abstract.In this study, computer modelling has been used to simulate and analyse the effects which non‐homogeneous stomatal conductance (patchiness) might have on the appearance of the macroscopic relationship between photosynthetic CO2assimilation (A) and intercellular CO2(Q) in leaves. The problem was formalized using the assumptions that (1) the biochemical model of Farquhar, von Caemmerer&Berry applies [Planta,149, 78–90 (1980)]; (2) that the parameters for the model are fixed for the period required to determine the relationship; (3) that the distribution of conductances in any leaf area is normal (but restricted to positive values); and (4) that the leaf is perfectly heterobaric. The model is interactive, allowing the user to explore–well beyond the conditions for which data are presented here–the effects of carboxylation capacity, photosynthetic electron transport rate and photorespiration over a range of possible conductances and degrees of patchiness. Regardless of the parameters used in the model, the results fail to predict the change in appearance of theA‐C1curve which has been attributed to patchiness in other reports: even when conductance varies with a standard deviation of twice its mean value, the effects on the curve are minor. The need for reconsideration of the methods currently used to interpret gas exchange studies is