Although exocytosis is likely to occur in plant cells, the control of this process is the subject of speculation, as no direct measurements of vesicle fusion to the plasma membrane have been made. We used the patch clamp technique to monitor the secretory activity of single aleurone protoplasts by measuring membrane capacitance (C(m)), while dialyzing the cytosol with different Ca(2+) containing solutions. Secretory activity increased with [Ca(2+)](i) approximately 1 muM. This demonstrates directly the existence of exocytosis in plant cells, and suggests that both plant and animal cells share common mechanisms (cytosolic Ca(2+)) for the control of exocytotic secretion.