A method is described for determining the space constant gamma of heart muscle strips using a sucrose gap technique. 2. The average value of gamma for frog atrial trabeculae was found to be nearly 700 mum. This value is nearly twice the length of the test gap (400 mum). Near the resting potential, the voltage non-uniformity should be about 10%. This was confirmed experimentally by comparing the membrane voltages recorded across the current-passing and voltage-recording sucrose gaps. 3. The non-uniformity during large depolarizations was calculated using a computer model. This model includes the inward-going rectification displayed by iK1 and the delayed rectification that occurs following depolarizations beyond -40mV. A single component of delayed rectification was included. 4. It is shown that even very large non-uniformities have relatively small effects on the shape of the activation curve and on the time course of onset or decay of current. 5. It is comcluded that the fast component of current decay described in a previous paper (Brown, Clark & Noble, 1976b) is not attributable to a non-uniformity artifact.
