Abstract The activities of enzymes catalysing glycollate oxidation, formate production and folate-dependent formate utilization were examined in the primary leaves of Hordeum vulgare cv Galt. Seedlings were grown for 6 days in darkness and then transferred to continuous light (500 μinsteins/m 2 per sec) for up to 5 days. Cell-free extracts of the primary leaves contained glycollate oxidase (EC 188.8.131.52), 10-formyltetrahydrofolate synthetase (EC 184.108.40.206), 5, 10-methylenetetrahydrofolate dehydrogenase (EC 220.127.116.11) and ability to enzymically decarboxylate glyoxylate. These activities increased during greening and at the end of the light treatment were 70–450% higher than etiolated controls. Greened primary leaves also incorporated [ 14C]formate at rates that were three- to four-fold higher than shown by etiolated leaves. The specific activity of 10-formyltetrahydrofolate synthetase was decreased by 20–35% when the leaves were greened in the presence of 10 mM hydroxysulphonate. This inhibitor also reduced the incorporation of [ 14C]formate by up to 45%. A potential flow of carbon from glycollate to 10-formyltetrahydrofolate via glyoxylate and formate was suggested by the data.