The effect of an aqueous fraction from the bulbs of Allium sativum (GE) was investigated in toad skin. When added to the inner (serosal) solution, GE caused a maximal reversible reduction of the transepithelial potential difference and short circuit current of 38% and 45%, respectively. When added to the outer (mucosal) solution, the effect was only partially reversible. Isaacson's amiloride test showed that GE decreased sodium potential (ENa.) and sodium conductance (GNa.). The net Na+ flux decreased due principally to a fall in Na+ flux in the active direction. GE decreased Na(+)-K+ ATPase activity in vitro. Partial replacement of sodium by choline in the outer solution reduced the effect of GE on the skin and substitution of normal Ringer's solution with isethionate Ringer's solution in the outer solution significantly enhanced the effect of GE on the skin. These results indicate that GE decreases active Na transport in the toad skin.