An increasing number of studies indicate that changes in cytosolic free Ca2+ ([Ca"],) mediate specific types of signal transduction in plant cells. Modulation of [Ca2+], is likely to be achieved through changes in the activity of Ca2+ channels, which catalyse passive influx of Ca2+ to the cytosol from extracellular and intracellular compartments. Voltage-sensitive Ca2+ channels have been detected in the plasma membranes of algae, where they control membrane electrical properties and cell turgor. These channels are sensitive to 1,4-dihydropyridines, which in animal cells specifically affect one class of voltage-regulated plasma membrane Ca2+ channel. Ca2+-permeable channels with different pharmacological properties have been found in the plasma membrane of higher plants. Recent evidence suggests the existence of two discrete classes of Ca2+ channel co-resident in the vacuolar membrane (tonoplast) of higher plants. The first is gated by inositol 1,4,5-trisphosphate, and bears a number of similarities to its animal counterpart which is located in the endoplasniic reticulum (ER). The second tonoplast CaZ+ channel is voltage-operated. However, the specific roles of these tonoplast channels in signal transduction have yet to be elucidated.