The photosynthetic response of stands of green peppers to light and CO2 is examined by means of various mathematical models. Several single leaf response equations are considered, from the simple and ubiquitous rectangular hyperbola, to more sophisticated forms incorporating photorespiration and the oxygen effect. By making the usual assumptions about stand structure and light penetration, mechanistic crop response equations arc derived for each of the different leaf response equations. First, as a purely empirical approach, the leaf response equations are applied directly to crop data, and it is concluded that the rectangular hyperbola with a term for dark respiration [eqn (2)] gives an adequate summary of crop response to light and CO2. Second, four mechanistic equations of crop response are applied to the data, and, although the results are equivocal, it is suggested that the simpler crop equations [eqns (15) and (17)] are satisfactory at the present time, and it is not yet possible to detect the results of photorespiration and the oxygen effect directly in the crop data.