Palytoxin (PTX) at concentrations higher than 10(-9) M increased tissue Na and decreased tissue K contents in the smooth muscle of rabbit aorta. The decrease in the tissue K content induced by PTX (10(-8)M) was complete within 1 hr. Saponin (1 mg/ml) and Triton X-100 (0.1% wt./vol.) also rapidly decreased the tissue K content. On the other hand, a high concentration of ouabain (10(-3)M) did not change the tissue K content within 1 hr of application, and the maximum loss of K was obtained after 6 hr. Loss of tissue Na into Na- and K-free solution from Na loaded muscle was accelerated by PTX (10(-8)M). The PTX-induced increase in loss of Na was inhibited in proportion to the decrease in the temperature from 37 to 10 degrees C, while the loss of Na in the absence of PTX was almost completely inhibited at 24 degrees C. Decrease in the wet weight of the muscle induced by hyperosmotic solution was inhibited by pretreatment with PTX (10(-8)M) or saponin (1 mg/ml) for 1 hr. PTX (10(-9) and 10(-8)M) had no effect on the ATP content of the muscle. However, PTX at concentrations above 10(-7)M reduced the ATP content, and a significant amount of ATP was detected in the incubation medium. Saponin (1 mg/ml) and Triton X-100 (0.2% wt./vol.) induced a release of Na from liposomes prepared with synthesized lecithin or total lipids of rabbit red blood cells. However, no Na leak was induced by PTX (10(-8)-10(-6)M) in these liposomes. These results suggest that PTX at low concentrations (10(-9)-10(-8)M) increases the membrane permeability of vascular smooth muscle cells to Na and K ions. At higher concentrations (10(-7)-10(-6)M). PTX seems to form pores which are permeable to a larger molecule like ATP. The results further suggest that the mode of action of PTX is different from that of saponin or detergent.