1. Intracellular recordings were made from rat locus coeruleus (LC) neurons in completely submerged brain slices. Trains of action potentials in LC neurons were followed by a prolonged post-stimulus hyperpolarization (PSH). If trains were elicited with depolarizing current pulses of sufficient intensity, PSH was composed of a fast, early component (PSHE) and a slow, late component (PSHL). PSH which followed trains elicited with lower intensity depolarizing current pulses consisted only of PSHL. 2. Both PSHE and PSHL were augmented by increasing the number of action potentials in the train and both were associated with an increase in membrane conductance. The reversal potential for PSHE was -108 mV and for PSHL it was -114 mV. 3. When a hybrid voltage clamp protocol was used, the current underlying PSH (IPSH) was observed to consist of an early, rapidly decaying component, IE, followed by a late, slower decaying component, IL. The time course of decay of IPSH was biexponential with the time constant of decay of IL more than one order of magnitude larger than the time constant of decay of IE. An increase in the concentration of external K+ shifted the reversal potentials for IE and IL in the depolarizing direction; the mean value of shift per tenfold increase in external K+ concentration was 57.1 mV for IE and 57.6 mV for IL. 4. Both PSHE and PSHL were inhibited by lowering the external Ca2+ concentration or by application of the Ca2+ channel blockers Cd2+ (200-500 microM) or nifedipine (100 microM). Intracellular injection of EGTA abolished both components of PSH. Increasing the external Ca2+ concentration augmented both PSH components. 5. Superfusion of dantrolene (25 microM) or ryanodine (20 microM) decreased the amplitude and duration of PSHL with much less effect on PSHE. 6. d-Tubocurarine (20-200 microM) selectively blocked PSHE with no effect on PSHL; this effect is the same as that of apamin which we have previously described. Superfusion with charybdotoxin (40 nM) or TEA (400 microM-1 mM) did not reduce PSHE or PSHL. 7. Inhibition of IA by 4-aminopyridine or 2,4-diaminopyridine also did not reduce either component of PSH. In fact, these agents slightly augmented both components of PSH; this effect was probably secondary to the prolongation of action potential duration. Superfusion of TEA in concentrations of 2-10 mM increased the size and duration of PSHL and increased the duration but decreased the size of PSHE.(ABSTRACT TRUNCATED AT 400 WORDS)