The rate of glycolate excretion by Coccochloris peniocystis Kütz. cells incubated under conditions of low bicarbonate concentration and high light intensity was linear for only the initial 15 min of incubation and no additional glycolate accumulated in the medium after 20 min. Excretion was maximal in cells grown on 5% CO2 in air when transferred to an incubation medium containing no added bicarbonate. The inhibitor INH (isonicotinyl hydrazide) had no measurable effect on the amount of glycolate released whereas HPMS (α-hydroxy-2-pyridyl methanesulfonate) stimulated excretion 3-fold. Cells transferred to air from growth on 5% CO2 in air increased in carbonic anhydrase activity, while a decrease occurred in the cells' ability to excrete glycolate. Cells grown on air and switched to 5% CO2 in air showed an increase in their ability to excrete glycolate with a concomitant decrease in carbonic anhydrase activity. Diamox, a specific inhibitor of carbonic anhydrase, was found to stimulate excretion with both airgrown and 5% CO2-grown cells which had been off 5% CO2 for approximately 30 min. The rate of carbon fixation by 5% CO2-grown cells put on air was found to rise over a 110 min period, corresponding to both the induction period of carbonic anhydrase and the period of decline in the ability of the cells to excrete glycolic acid. These results suggest that the absence of carbonic anhydrase in 5% CO2-grown cells causes a stimulation of glycolate excretion when these cells are transferred to a low bicarbonate medium, because of an increased rate of glycolate formation due to the oxidation of ribulose diphosphate by molecular oxygen at low internal CO2 concentrations.