This study was designed to clarify the mechanism of the inhibitory effect of forskolin on contraction, cytosolic Ca2＋ level ([Ca2＋]i), and Ca2＋ sensitivity in guinea pig ileum. Forskolin (0.1 nM∼10ՌM) inhibited high K＋ (25 mM and 40 mM)- or histamine (3ՌM)-evoked contractions in a concentration- dependent manner. Histamine-evoked contractions were more sensitive to forskolin than high K＋- evoked contractions. Spontaneous changes in [Ca2＋]i and contractions were inhibited by forskolin (1ՌM) without changing the resting [Ca2＋]i. Forskoln (10ՌM) inhibited muscle tension more strongly than [Ca2＋]i stimulated by high K＋, and thus shifted the [Ca2＋]i-tension relationship to the lower-right. In histamine-stimulated contractions, forskolin (1ՌM) inhibited both [Ca2＋]i and muscle tension without changing the [Ca2＋]i-tension relationship. In Ձ-toxin-permeabilized tissues, forskolin (10ՌM) inhibited the 0.3ՌM Ca2＋-evoked contractions in the presence of 0.1 mM GTP, but showed no effect on the Ca2＋-tension relationship. We conclude that forskolin inhibits smooth muscle contractions by the following two mechanisms: a decrease in Ca2＋ sensitivity of contractile elements in high K＋- stimulated muscle and a decrease in [Ca2＋]i in histamine-stimulated muscle.