The level of malate in the epidermis from illuminated leaves of Vieia ]aba was greater than in that from dark-treated leaves. A difference in the malate level was still detected after the epidermis had been treated by rolling so that only the guard cells remained alive. The results suggest that malate may accumulate in guard cells on illumination. In subsequent experiments, stomatal apertures were measured, and potassium as well as malate was analysed in extracts of epidermis. In illuminated leaves, the potassium content of rolled epidermis increased from about 90 to about 335 picoequivalents mm -2 of epidermis while malate increased from about zelx) to about 71 pmoles mm -2 and the stomata opened; in dark-treated leaves, the potassium content of rolled epidermis decreased slightly, the malate level remained about zero, and the stomata showed very slight further closure. The measured increase in potassium is likely to represent an increase in potassium concentration in the guard cells of about 0.4 Eq 1-1 with stomatal opening; the increase in malate could correspond to 0.23 Eq1-1 (with respect to potassium) in the guard cells. Thus, malate accumulating in guard cells could balance about half of the potassium taken up by guard cells when stomata open in the light.