Fusicoccin and abscisic acid can modify the movements of some nyctinastic plants. This paper reports the effects of these two compounds on the concentrations of malic acid in pulvini of Phaseolus coccineus L. in the transition period of two phases of the circadian rhythm. The concentrations of malic acid in the two parts of the pulvinus, extensor and flexor, depend upon the phase of leaf movements. Abscisic acid and fusicoccin significantly affected the content of malic acid in leaves compared with the control. In the two phases of the diurnal cycle investigated, abscisic acid markedly reduced the production of malic acid in the flexor and extensor regions of the pulvinus, and treatment with fusicoccin produced small but distinct increases in the concentration of malic acid in the same regions. The results are discussed with particular reference to the effect of abscisic acid and fusicoccin on the activity of the H+ pump associated with the control of turgor in cells.