Abstract. Rapid-cooling pulses to non-stressful temperatures cause strong, transient depolarizations in cortical cells of cucumber roots. The amplitudes of these electrical responses are graded according to the rate and amplitude of the cooling pulse. Such graded potentials are typical of sensory processes and indicate that plants possess the ability to sense temperature change. La3+, a blocker of Ca2+ channels, and ethylene glycol bis-(β-aminoethyl ether) N,N,N′,N′-acetic acid (EGTA), a Ca2+ chelator, inhibit the electrical responses elicited by rapid-cooling pulses. High external [Ca2+] enhances them. These results indicate the involvement of a plasma membrane-associated Ca2+ channel in the process of temperature sensing by plants. Calmodulin antagonists prolong the repolarization phase of the electrical responses, suggesting a role for calmodulin in the recovery from stimulation.