In lobster olfactory receptor neurons (ORNs), depolarizing responses to odorants and current injection are accompanied by the development of an afterhyperpolarization (AHP) that likely contributes to spike-frequency adaptation and that persists for several seconds after termination of the response. A portion of the AHP can be blocked by extracellular application of 5 mM CsCl. At this concentration, CsCl specifically blocks the hyperpolarization-activated cation current (Ih) in lobster ORNs. This current is likely to be active at rest, where it provides a constant, depolarizing influence. Further depolarization deactivates Ih, thus allowing the cell to be briefly hyperpolarized when that depolarizing influence is removed, thus generating an AHP. Reactivation of Ih would terminate the AHP. The component of the AHP that could not be blocked by Cs+ (the Cs(+)-insensitive AHP) was accompanied by decreased input resistance, suggesting that this component is generated by increased conductance to an ion with an equilibrium potential more negative than the resting potential. The Cs(+)-insensitive AHP in current clamp and the underlying current in voltage clamp displayed a reversal potential of approximately -75 mV. Both EK and ECl are predicted to be in this range. Similar results were obtained with the use of a high Cl- pipette solution, although that shifted ECl from -72 mV to -13 mV. However, when EK was shifted to more positive or negative values, the reversal potential also shifted accordingly. A role for the Ca(2+)-mediated K+ current in generating the Cs(+)-independent AHP was explored by testing cells in current and voltage clamp while blocking IK(Ca) with Cs+/Co(2+)-saline. In some cells, the Cs(+)-independent AHP and its underlying current could be completely and reversibly blocked by Cs+/Co2+ saline, whereas in other cells some fraction of it remained. This indicates that the Cs(+)-independent AHP results from two K+ currents, one that requires an influx of extracellular Ca2+ and one that does not. Collectively, these findings indicate that AHPs result from three phenomena that occur when lobster ORNs are depolarized: 1) inactivation of the hyperpolarization-activated cation current, 2) activation of a Ca(2+)-mediated K+ current, and 3) activation of a K+ current that does not require influx of extracellular Ca2+. Roles of these processes in modulating the output of lobster ORNs are discussed.