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School of Biological Sciences, University of East Anglia, Norwich.

1. The effects of the sulphydryl-complexing reagent p-chloro-mercuriphenyl sulphonate (pCMPS) on membrane voltage and electrical conductance were studied on the isolated frog lens. 2. At low concentrations (0.1-50 microM) pCMPS induced a rapid and graded hyperpolarization of the lens membrane potential which saturated at -97 mV. 3. The lens conductance also showed a graded increase, but the initial changes were apparent only at concentrations above 1 microM. 4. Decreasing the external potassium concentration from 2.5 to 0.5 mM had little effect on the membrane potential in the absence of pCMPS, but increased the voltage from -97 to -110 mV when pCMPS was present. 5. Quinine (300 microM) had no effect when added in control solution, but depolarized the membrane potential and decreased the conductance when added to a pCMPS-treated preparation. 6. These data suggest that pCMPS activates voltage-sensitive potassium channels that are quiescent at the frog resting potential in control solution. 7. At pCMPS concentrations greater than or equal to 100 microM, the initial hyperpolarization is followed by a marked but slow depolarization of the membrane potential and a further increase in lens conductance. These data suggest that non-specific cation channels are activated in this case. 8. Cysteine (5 mM) added to a pCMPS-treated lens leads to a rapid recovery of membrane potential and conductance to near their resting values whether the lens had previously been exposed to low or high concentrations of pCMPS. 9. All of these changes in lens voltage and conductance occurred without apparent alteration in the lens internal sulphydryl content.