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1. The outward currents underlying delayed rectification have been studied in frog auricular trabeculae with the double sucrose gap method.

2. The results obtained in our experimental conditions allow us to discard the hypothesis of an accumulation of ions during the flow of the outward current.

3. In most of the cases the delayed current is composed of two components similar to those previously described by Noble & Tsien (1969a, b).

4. The first component I₁ is activated with a time constant of about 0·3 sec at the resting potential (E = - 70 mV) and of about 0·75 sec at E = - 5 mV. The steady-state value of the degree of activation (x₁) varies from 0 at E = - 90 mV to 1 at E = + 20 mV. The reversal potential is near E = - 80 mV; this current seems to be mainly carried by potassium ions.

5. The second component I₂ is very slowly activated, with a time constant of 3·5 sec at E = - 70 mV and of 5 sec at E = - 5 mV. The steady-state value of the degree of activation (x₂) varies from 0 at E = - 40 mV to 1 at E = + 25 mV. The reversal potential is near E = - 60 mV; this current is less specific than the first component.

6. In some preparations the delayed current is composed of only one component. This current is activated with a time constant of 0·6 sec at the resting potential and of 1·5 sec at E = - 5 mV. The steady-state value of the degree of activation is 0 at E = - 90 mV and 1 at E = + 30 mV. The reversal potential is between E = - 70 and E = - 60 mV.

7. The role of these currents in the processes of repolarization of normal and prolonged action potentials, and in the pace-making activity is discussed.