Abstract The effects of carbon dioxide concentration (CO 2) in the range 300–1100 μmol mol −1 on leaf conductance ( g) and rate of crop transpiration ( E) of sweet pepper ( Capsicum annuum L.) were investigated in spring 1990. In two greenhouse compartments (154 m 2) that were simultaneously exposed to different CO 2 levels, leaf conductance of the upper leaves was measured with a steady state diffusion porometer and crop transpiration rates were measured with three weighing lysimeters per greenhouse compartment. Multiple regression equations, describing the effects of photosynthetic active radiation (PAR), vapour pressure deficit (VPD)-leaf-air, CO 2 and optionally leaf temperature on g, were fitted to the measured data. The fitted regression curves demonstrated that 100 μmol mol −1 increase in CO 2 reduced g by about 3%, at any level of CO 2, VPD and PAR, if VPD and PAR would remain constant. Measured rates of crop transpiration were highly correlated to radiation and were in reasonable accordance with the Penman-Monteith combination equation. With this equation it was estimated that a 10% decrease in g would reduce E by 1.5–3% at high levels of g (high radiation) and by 4–7% at low g (dark weather), at least if VPD would remain constant. In a greenhouse-crop system, however, owing to thermal and hydrologic feedbacks, an increase in CO 2 leads to a considerable increase in VPD-leaf-air. This enforces the effect of CO 2 on g and counteracts the effect of CO 2 on E, because the driving force for transpiration is enhanced. Thus, in general the apparent response of g to changes in CO 2 is far greater than the mentioned percentage, whereas the apparent response of E is relatively small.