ABSTRACT-To characterize the calcium (Ca")-releasing effects of histamine and GTPrS, the drug-in duced tension developments were measured in (3-escin-treated skinned longitudinal smooth muscle of guinea pig ileum. Intracelluar Ca2+ stores were loaded with Ca" by incubating the muscle for 10 min in a Ca" containing solution. Histamine (10-100 pM), applied after Ca" -loading, produced a transient rise in ten sion. The effect of histamine was not preserved after treatment with 20 mM caffeine, a Ca2+-store releaser. The effect of histamine was potentiated by GTP; inhibited by GDP(3S, an antagonist of GTP for binding to G-proteins; or heparin, an antagonist of inositol 1,4,5-trisphosphate (IP3) for binding to its receptor; and mimicked by IP3. When GTPrS (20 ttM) was applied and continued to be present for 15 min, a transient rise in tension followed by a small, sustained rise in tension was elicited. The effect of GTPrS was completely inhibited by GDP(3S. The initial, transient component of the biphasic GTPrS response was abolished or markedly inhibited after treatment with caffeine, heparin or the calcium ionophore A23187. The present results suggest that histamine and GTPrS cause a release of Ca 21 from caffeine-sensitive stores which is mediated by IP3 formed through a G-protein-coupled mechanism. The GTPrS-induced Ca 2+ release is not considered to involve such an IP3-independent process as described in chemically-skinned arterial muscle.