It appears that, in smooth muscle, there are two distinct types of Ca2+ channel, a voltage-dependent Ca2+ channel and a receptor-linked Ca2+ channel. The former is activated by decreases in membrane potential and the latter is regulated by drug-receptor interactions. These Ca2+ channels exist as separate channels in the aorta of the adult rabbit and of some rat strains; organic Ca2+ antagonists and sodium nitroprusside selectively inhibit each of these two respective channels. By examining the effects of these two specific antagonists, characterization of the Ca2+ channels in other types of smooth muscle was attempted. In a wide variety of vascular smooth muscle (except aorta of above-mentioned animals), however, these two Ca2+ channels showed some sensitivity to both types of inhibitor. It is proposed that in most types of vascular smooth muscle as well as in gastric fundus and corpus, the two types of Ca2+ channel are functionally not completely separated. These subtypes of Ca2+ channels are at least partly sensitive to both organic Ca2+ antagonists and sodium nitroprusside. Intestinal, genital, and tracheal smooth muscles also seem to have two types of Ca2+ channel. These subtypes of Ca2+ channels are, however, sensitive only to organic Ca2+ antagonists and are not affected by sodium nitroprusside.