Harmaline stimulates transport of sodium through the isolated frog skin when the alkaloid is added at low concentration (10(-5) to 10(-6) M) to the external medium. In the same conditions, it inhibits in vivo transport of sodium through the gills of the crayfish. It is proposed that these effects do not involve a (Na + K) ATPase but are the result of an interaction between harmaline and the sites of penetration of sodium on the external surface.