Cultured Kenyon cells from the mushroom body of the honeybee, Apis mellifera, show a voltage-gated, fast transient K+ current that is sensitive to 4-aminopyridine, an A current. The kinetic properties of this A current and its modulation by extracellular K+ ions were investigated in vitro with the whole cell patch-clamp technique. The A current was isolated from other voltage-gated currents either pharmacologically or with suitable voltage-clamp protocols. Hodgkin- and Huxley-style mathematical equations were used for the description of this current and for the simulation of action potentials in a Kenyon cell model. Activation and inactivation of the A current are fast and voltage dependent with time constants of 0.4 +/- 0.1 ms (means +/- SE) at +45 mV and 3.0 +/- 1.6 ms at +45 mV, respectively. The pronounced voltage dependence of the inactivation kinetics indicates that at least a part of this current of the honeybee Kenyon cells is a shaker-like current. Deactivation and recovery from inactivation also show voltage dependency. The time constant of deactivation has a value of 0.4 +/- 0.1 ms at -75 mV. Recovery from inactivation needs a double-exponential function to be fitted adequately; the resulting time constants are 18 +/- 3.1 ms for the fast and 745 +/- 107 ms for the slow process at -75 mV. Half-maximal activation of the A current occurs at -0.7 +/- 2.9 mV, and half-maximal inactivation occurs at -54.7 +/- 2.4 mV. An increase in the extracellular K+ concentration increases the conductance and accelerates the recovery from inactivation of the A current, affecting the slow but not the fast time constant. With respect to these modulations the current under investigation resembles some of the shaker-like currents. The data of the A current were incorporated into a reduced computational model of the voltage-gated currents of Kenyon cells. In addition, the model contained a delayed rectifier K+ current, a Na+ current, and a leakage current. The model is able to generate an action potential on current injection. The model predicts that the A current causes repolarization of the action potential but not a delay in the initiation of the action potential. It further predicts that the activation of the delayed rectifier K+ current is too slow to contribute markedly to repolarization during a single action potential. Because of its fast activation, the A current reduces the amplitude of the net depolarizing current and thus reduces the peak amplitude and the duration of the action potential.