Wild type and calmodulin mutants (cam) of Paramecium tetraurelia were examined for cold-sensitive responses. Among mutants tested, cam¹² and cam¹³ mutants, which have substitutions in N-terminal lobe of calmodulin molecule, reduced both responses in the swimming and the membrane potential. Under voltage clamp conditions, the cooling stimulus to the wild type cell induced a transient inward current whose amplitude increased with the rate of temperature drop. The cam¹² cell did not induce inward currents in response to cooling with a rate slower than - 0.4°C/s. The reduced current response of cam¹² mutant was restored by an external application of a phosphodiesterase inhibitor, theophylline. Also, an intracellular injection of hydrolysis-resistant cyclic nucleotides, either 8-bromoadenosine 3', 5'-cyclic monophosphate (8-Br-cAMP) or 8-bromoguanosine 3, 5'-cyclic monophosphate (8-Br-cGMP), restored the current response. Such restoration was accompanied by shifts of the resting potential to hyperpolarized levels and by an increase in the membrane conductance. The results suggest the possibility that calmodulin and cyclic nucleotide regulate K⁺ channels responsive to the cooling stimulus.