The ability of dithiol compounds with different spacing between the thiol (SH) groups to protect against and reversed the action of alloxan on islet tissue permeability, and their ability to inhibit the reaction between alloxan and glutathione (which results in the formation of a compound with a 305 nm absorption maximum) have been examined. Treatment of toadfish islet slices with alloxan markedly increased their permeability to D-mannitol-1-14C, which normally is restricted to the extracellular space. Pretreatment of the slices with 2,3-dimercaptopropanol (BAL) or 1,4-dimercaptobutane (DMB) before treatment with alloxan completely protected them against this action of alloxan, whereas 1,5-dimercaptopentane (DMP) and 1,6-dimercaptohexane (DMH) partially protected and 1,8-dimercapto-octane (DMO) had no effect. When islets were first treated with alloxan and then treated with the dithiols, BAL and DMB reverse the action of alloxan to the greatest extent, DMP was less effective, and DMH had no effect. The effect of the dithiols on the reaction between alloxan and glutathione was consistent with their effects on the alloxan-induced increase in islet permeability; BAL and DMB were the strongest inhibitors, DMP and DMH inhibited to a lesser degree and DMO did not inhibit. These studies support the hypothesis that alloxan damages islet cell membranes by reacting with membrane dithiols formed by two SH groups which are relatively close together.