The calcium antagonist prenylamine which had previously been found to inhibit fully the calmodulin myosin light chain kinase systems in concentrations of 200 mumol/l has now been found to inhibit the calmodulin-dependent contraction of skinned smooth muscle (guinea pig taenia coli). The inhibition by prenylamine is itself calcium-dependent. At maximum activation with Ca2+ and calmodulin, little inhibition is observed. Verapamil as well as prenylamine in small concentrations (less than 10 mumol/l) have been found to increase the rate of calcium-induced isometric tension development.