Under conditions where glycolate synthesis was inhibited at least 50% in tobacco (Nicotiana tabacum L.) leaf discs treated with glycidate (2,3-epoxypropionate), the ribulose diphosphate carboxylase activity in extracts and the inhibition of the activity by 100% oxygen were unaffected by the glycidate treatment. [1-(14)C]Glycidate was readily taken into leaf discs and was bound to leaf proteins, but the binding occurred preferentially with proteins of molecular weight lower than ribulose diphosphate carboxylase. Glycidate added to the isolated enzyme did not inhibit ribulose diphosphate carboxylase activity or affect its inhibition by 100% O(2). Thus, glycidate did not inhibit glycolate synthesis by a direct effect on ribulose diphosphate carboxylase/oxygenase.NADH-glyoxylate reductase and phosphoglycolate phosphatase activities were also unaffected in extracts of leaf discs supplied with glycidate, but NADPH-glyoxylate reductase was inhibited about 35% in such extracts. Addition of 10 mm glycidate to isolated NADPH-glyoxylate reductase inhibited the activity about 25% after 15 minutes of reaction.The small inhibition of NADPH-glyoxylate reductase by glycidate may help to explain the increase in glyoxylate concentration found in glycidate-treated leaf discs. Increasing the glyoxylate pool size in leaf discs has been shown to effectively block glycolate synthesis and photorespiration and increase net photosynthesis. Thus, the similar effects brought about by glycidate in leaf discs can be attributed to indirect effects of metabolic regulation.