The venom of the black mamba contains a 60-amino acid peptide called calciseptine. The peptide has been fully sequenced. It is a smooth muscle relaxant and an inhibitor of cardiac contractions. Its physiological action resembles that of drugs, such as the 1,4-dihydropyridines, which are important in the treatment of cardiovascular diseases. Calciseptine, like the 1,4-dihydropyridines, selectively blocks L-type Ca2+ channels and is totally inactive on other voltage-dependent Ca2+ channels such as N-type and T-type channels. To our knowledge, it is the only natural polypeptide that has been shown to be a specific inhibitor of L-type Ca2+ channels.