Reduction of 1 (verapamil) afforded amine 2, which was converted with thiophosgene to isothiocyanate 3, a chemoaffinity ligand for Ca2+ channels. Compound 3 showed concentration-dependent negative inotropic effects in rat right myocardial ventricular strips, EC50 = (4.56 +/- 3.40) X 10(-6) M (mean +/- SD), being slightly less potent than 4 (gallopamil), EC50 = (1.95 +/- 1.22) X 10(-6) M. It displaced [3H]gallopamil in rat myocardial membranes, IC50 = (3.42 +/- 2.51) X 10(-7) M, approximately equipotent with 1. It showed irreversible antagonism of [3H]gallopamil binding when preincubated at 10(-5) M; only 25% of [3H]gallopamil binding vs. control was observed. This agent may be a useful chemoaffinity ligand to aid in characterization of Ca2+ channels.