Rhythmic up-down movements were studied in the lateral leaflets of Desmodium gyrans (L.F.) DC. These were recorded with a video-computer system, whereby the digital video signals from a CCD camera were processed with special software. Under control conditions ($24^{circ}C;and;0.1nM;cm^{-2} s^{-1}$ of stable, dim light), the average period of lateral leaflet movement was 3.5 min. In the presence of light stimuli (for 2 min), those leaflets always moved toward the light, regardless of where it was applied to any axial part of the pulvinus. The strongest effect was manifested by a reduced amplitude of movement and, thus, a shortened period, which could be up to ${sim}43%$ less under moderate light intensity $(5nM cm^{-2} s^{-1})$. Oscillations regained their original regularity over ${sim}10$ cycles after the light stimulus was removed. In addition, these oscillations temporarily disappeared after long exposure $({sim}10 min)$ under moderate light, or when the leaflets were quickly exposed to a higher intensity $({sim}12nM;cm^{-2}s^{-1})$. Therefore, we have now demonstrated that light can affect physiological parameters that are involved in the control of oscillations.