1. Outward directed membrane currents have been studied in voltage clamp experiments on isolated neural somata of the marine gastropod Anisodoris.2. Stepping the membrane potential from a hyperpolarized level to a value in the neighbourhood of resting potential (-35 to -50 mV at 5 degrees C) results in an outward current transient, I(A), which is apparently carried by potassium ions.3. The peak amplitude of I(A) is dependent upon both the holding voltage level and the test step voltage while the time courses of development and decay are independent of, or only slightly dependent on, these parameters.4. The developing and decaying phases of I(A) are approximated by exponentials, leading to time constants for development of 10-25 msec and for decay of 220-600 msec over the aggregate of cells studied (data at 5 degrees C). Q(10) for the processes is approximately 3.5. It is concluded that the transport mechanism for I(A) is at least operationally distinct from the mechanism underlying delayed outward current, I(K).