Experiments were designed to assess the mode of action of palytoxin (PTX), isolated from Palythoa tuberculosa, on mechanically denervated rabbit aortic strips. PTX induced a sustained contraction in the muscle dose dependently. The contraction was irreversible. In the depolarized aorta, PTX did not induce a contraction whereas norepinephrine (NE) did. Removal of calcium from the bathing medium prevented PTX and high K+ contractions, whereas phasic responses were elicited by NE. D600 also inhibited the contraction induced by PTX or high K+ but had a lesser effect on that induced by NE. Sodium nitroprusside inhibited only the effect of NE. PTX increased dose dependently the 45Ca uptake of a fraction not removable by La3+ treatment and the increase was inhibited by D600. High K+ also increased the 45Ca uptake although NE did not. It is suggested that PTX increased Ca2+ influx into the smooth muscle cell to cause a contraction, which may be analogous to the action of high K+.