The synchronized 12:12 h (L:D) primary leaf movement of Phaseolus vulgaris was profoundly altered by a calcium chelator (EGTA 10 mM). This effect depended strongly on the phase at which this chemical was administered. When EGTA was sprayed on the leaf 8 h after lights-on the upward movement was advanced by 3-6 h. At a critical time point in the middle of the night, (around 17 h after the light-on, 5 h after light-off respectively) EGTA suppressed circadian rhythmicity and promoted an ultradian rhythm. The period of this ultradian rhythm was significantly longer (4.04 ± 0.09 h) in light than in darkness (3 .20 ± 0.13 h), and the amplitude was also higher in light than in darkness.