The mechanism responsible for the inhibition of net carbon exchange (NCE) which was reported previously (DR Geiger et al. 1986 Plant Physiol 82: 468-472) was investigated by applying glyphosate [N-(phosphonomethyl)glycine] to exporting leaves of sugar beet (Beta vulgaris L.). Leaf internal CO(2) concentration (C(i)) remained constant despite decreases in stomatal conductance and NCE following glyphosate treatment, indicating that the cause of the inhibition was a slowing of carbon assimilation rather than decreased conductance of CO(2). Throughout a range of CO(2) concentrations, NCE rate at a given C(i) declined gradually, with the time-series of response curves remaining parallel. Gas exchange measurements revealed that disruption of chloroplast carbon metabolism was an early and important factor in mediating these glyphosate effects, perhaps by slowing the rate of ribulose bisphosphate regeneration. An increase in the CO(2) compensation point accompanied the decrease in NCE and this increase was hastened by stepwise lowering of the ambient CO(2) concentration. Eventually the CO(2) compensation point approached the CO(2) level of air and the difference between internal and external CO(2) concentrations decreased. In control and in glyphosate-treated plants, both carbon assimilation and photorespiration at atmospheric CO(2) level were inhibited to a similar extent of air level of O(2). Maintaining leaves in low O(2) concentration did not prevent the decline in NCE rate.