Both human and guinea pig gallbladder strips were tonically contracted by supramaximal concentrations of histamine in vitro. Guinea pig gallbladder strips were also tonically contracted by potassium, with no spontaneous relaxation after 5 min. The amplitude of the contractions caused by histamine decreased spontaneously only by 34 ± (SD) 10% after 5 min in guinea pig strips and by 25 ± 7% after 5 min in human strips. Both oxytocin and 8-l-lysine-vasopressin dose-dependently relaxed almost completely contractions caused by histamine and potassium in guinea pigs. In humans, only 8-l-lysine-vasopres-sin decreased the amplitude of histamine contractions. In both species, neither oxytocin nor 8-l-lysine-vasopressin affected the basal tone of strip