A radioreceptor assay for calcium channel antagonist drugs described here is based on the ability of these drugs to affect 3H-nitrendipine binding to calcium channels. All the known calcium channel antagonists may be assayed in this manner. The assay can detect 10-100 nM (4 - 40 ng/ml) nimodipine, 10-100 nM (3.5 - 35 ng/ml) nifedipine, 3-30 microM (1.2 - 12 micrograms/ml) prenylamine, 0.1 - 1.0 microM (49 - 490 ng/ml) verapamil and 3-30 microM (1.2 - 12 micrograms/ml) diltiazem. These values cover the range of concentrations of calcium channel antagonists that are clinically important. As the radioreceptor assay detects active metabolites as well as the parent drugs, it should prove a useful adjunct in cardiovascular therapy. The method is more reproducible, simpler and less expensive than other methods such as high pressure liquid chromatography.