Inward Ca2+ currents exist in many excitable tissues and are linked to regulation of several cellular processes such as cyclic GMP formation1–4. In Paramecium, a graded Ca2+/K+ action potential regulates swimming behaviour5–7. Voltage-gated Ca2+ channels localized in the excitable ciliary membrane8,9 conduct a depolarizing influx of Ca2+ and translate changes in membrane potential into a transient Ca2+ signal which triggers ciliary reversal, that is, backward swimming5,6. A guanylate cyclase that is activated specifically by Ca2+ has already been characterized in the ciliary membrane10. By using behavioural mutants of Paramecium with reduced6,11,12 or exaggerated13,14 Ca2+ currents, we now demonstrate in an intact animal a direct link between the voltage-gated inward Ca2+ current and an elevation of cyclic GMP levels. Although the increased cyclic GMP level does not directly influence swimming behaviour, the combination of electrophysiology, biochemistry and genetics possible with Paramecium offers an opportunity of identifying the role of cyclic GMP levels in cell behaviour.