The level of malate in the epidermis from illuminated leaves of V. faba 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. Results suggested that malate might accumulate in guard cells on illumination. In subsequent experiments, stomatal apertures were measured, and K as well as malate was analysed in extracts of epidermis. In illuminated leaves, the K content of rolled epidermis increased from about 90 to about 33.5 peq/mm2 epidermis, while malate increased from about 0 to about 71 pM/mm2 and the stomata opened; in dark-treated leaves, the K content of rolled epidermis decreased slightly, the malate level remained about zero, and the stomata showed very slight further closure. The measured increase in K was likely to represent an increase in K concentration in the guard cells of about 0.4 eq/l with stomatal opening; the increase in malate could correspond to 0.23 eq/l (with respect to K) in the guard cells. Malate accumulating in guard cells could thus balance about half the K taken up by guard cells when stomata open in the light.