A voltage-clamp method was used to examine the effects of zinc ions on the delayed potassium currents and also the slowly activating potassium currents that are turned on by depolarizing the membrane of frog sartorius muscle fibres. 2. In a control solution, the delayed potassium conductance had a maximum value of 17-8 +/- 2-5 mmho.cm-2. The reversal potential for the currents was -76-9 +/- 2-5 mV. The membrane potential where ninfinity had the value 0-5 was -49 mV. 3. The major effect of zinc ions was to slow the delayed potassium currents. The value of taun was increased approximately tenfold in 0-1 mM zinc. Zinc does not alter the effective valency of the gating particles of the potassium channel, but the conductance was shifted to more positive membrane potentials: in 0-1 mM zinc, the membrane potential where ninfinity had the value 0-5 was -32 mV. 4. Zinc ions, at a concentration of 0-1 mM, also reduced the maximum potassium conductance by about 60% to 7-3 +/- 0-8 mmho, cm-2; they did not alter the reversal potential of the currents, which had a value in 0-1 mM zinc of -74-6 +/- 1-5 mV. 5. Zinc ions had little or no effect on the rate of inactivation of the potassium currents. 6. Zinc ions had little effect on the conductance attributable to the slowly activating potassium system. In 0-01 mM zinc this conductance had a value at 0 to +10 mV of 1-25 +/- 0-29 mmho.cm-2. Zinc did not alter the reversal potential of the slow potassium currents from the value of -85 +/- 1-6 mV in the absence of zinc and had no effect on the time course of the turn-off of these currents at -60 mV. 7. The delayed potassium currents obtained in 0-002 and 0-01 mM zinc could not be fitted exactly with a simple fourth order equation, but were well fitted by a model proposing that zinc ions slow the opening and closing of the gating mechanism to one tenth the normal rate when they bind to the gating molecule. If the binding sites are not saturated, those gating molecules that do not bind zinc are assumed to be quite unaltered in their properties, though the potential dependence of their rate constants alphan and betan was assumed to be shifted to more positive levels. In one fibre in 0-01 mM zinc, the model fitted the currents best if 50% of the gating molecules bound zinc.
