Signal transduction events that lead to circadian control of physiology are poorly understood. Signalling elements that could transmit time information include transcription factors, reversible phosphorylation, and changes in the concentration of cytosolic free calcium ([Ca(2+)](cyt)). [Ca(2+)](cyt) oscillates with a circadian rhythm in Arabidopsis and Nicotiana, but does not have a defined role in circadian signalling. [Ca(2+)](cyt) oscillations with shorter periods encode specific signals in several cell types, therefore circadian [Ca(2+)](cyt) oscillations provide a potential mechanism for signalling time information. Cell types such as stomatal guard cells and legume pulvini represent attractive model systems for dissecting circadian Ca(2+) signalling.