In an attempt to categorize the various early transient K+ currents (A- type K+ currents) present in mature neurons, we have explored these currents in the identified neurons in the abdominal ganglion of the mollusk, Aplysia californica. Three distinct types of A-type K+ currents (IAfast, IAslow, and IAdepol) were found. The activation and the steady-state inactivation properties of two of these currents, IAfast and IAslow, were similar to conventional A-type K+ currents. By contrast, those of the third current, IAdepol, were shifted to more depolarized potentials. Whereas the decay time constants of IAfast were voltage dependent, those of IAslow and IAdepol were almost voltage independent. The recovery from inactivation of IAfast and IAslow was much faster than that of IAdepol. In addition, IAdepol was more sensitive to 4-aminopyridine (4-AP) than other currents and was almost completely blocked by 1 mM 4-AP. All the currents were depressed by forskolin or 1,9-dideoxyforskolin, but not by cAMP analogs. None of these currents were blocked by external tetraethylammonium (50 mM). These results indicate that there are at least three subtypes of A-type K+ currents in the Aplysia CNS. Of these, two currents, IAfast and IAslow, are conventional A-type K+ currents, whereas the third current, IAdepol, is a novel early transient K+ current.