1 Harmaline, together with certain hallucinogenic alkaloids of the same group (harmine, 2 methyl harmin) stimulates sodium transport across the in vitro skin of Rana esculenta when it is added to the external medium at a low concentration (0.1 mM). This effect is due to an increase of the sodium influx, and is reversed by washing. It is suggested that harmaline intervenes at the sodium penetration sites at the external face of the transport compartment. 2 At a higher concentration (5 mM) added to the internal medium harmaline inhibits sodium net absorption. The inhibition is due mainly to an increase of the efflux, while the influx may be either inhibited or increased. Under these conditions the influx becomes insensitive to amiloride. It is suggested that the inhibition of sodium transport is the result of harmaline interfering with a transport ATPase, and also that harmaline induces new sites for the passage of sodium.