Abstract: The characteristics of³H-d-cis-diltiazem binding sites in cardiac sarcolemmalmembrane of rats and effects of diltiazem and cyproheptadine(Cyp ) on these binding sites werestudied.³H-d-cis-diltiazem,a benzothiazepine calcium antagonist, was shown to be bound to crudecardiac sarcolcmmal membrane of rats in a specific and saturable manner with a K_D= 84 nmol·L^-1and a receptor site density (maximum binding) of 0. 279 pmol· mg protein^-1. Diltiazem and Cyp,an antiserotonin-antihistaminic agent with calcium channel blocker activity, was found to completelyinhibit the binding with Ki values of 102 nmol. L^-1 and5.5umol·L^-1, respectively. The resultsdemonstrate the existence of³H-d-cis-diltiazem binding sites in cardiac sarcolemmal membrane ofrats and also suggest that the calcium channel blocker activity of Cyp on cardiac cells may be related,at least in part, to its binding to the ³H-d-cis-diltiazem binding sites in cardiac sarcolemmalmembranes.