Effects of diltiazem on K-induced contracture of the smooth muscle were examined in the isolated taenia coli and stomach strip of the guinea pig. Results were compared with those of lanthanum ion (La(3+)). K-contracture was evoked by isotonic K-Krebs solution. La(3+) inhibited K-contracture of the taenia coli and effects on the phasic response were found to be more remarkable than on the tonic response. After the removal of La(3+), the phasic response almost regained its normal magnitude, whereas the tonic response was significantly augmented. In contrast to the effects of La(3+), diltiazem inhibited the tonic response more markedly than the phasic response in both taenia coli and stomach strip. Inhibition of the phasic response was elicited, in association with decrease in the frequency of spike discharge during rapid depolarization of the membrane. The effects of diltiazem were completely reversed after washing out the compound. It is assumed that although both La(3+) and diltiazem inhibit the transmembrane influx of Ca(2+), additional mechanisms may differ.