A long-lasting afterhyperpolarization (AHP) follows the antidromic or current-induced action potential of sympathetic preganglionic neurons (SPNs) studied in slices of cat spinal cord maintained in vitro. Duration and amplitude of the AHP that follows a single spike were 2.8 +/- 0.3 s and 16.0 +/- 0.7 mV (mean +/- SE), respectively. In most cases two components could be distinguished, an initial faster and usually larger component [fast (F) AHP] followed by a slowly decaying component [slow (S) AHP]. An increase in membrane conductance was associated with the AHP. The amplitude of both components increased with membrane depolarization and decreased with hyperpolarization. Both fast and slow component were nullified at a voltage of -90 mV in 3.6 mM K+. Peak AHP amplitude decreased as K+ was increased from 1.5 to 7.0 mM. The null point for both fast (F) AHP and slow (S) AHP shifted in the depolarizing or hyperpolarizing direction when K+ was increased or decreased, respectively. These data suggest that an increase in K+-conductance is the mechanism underlying the AHP. The two components of the AHP could be separated by their differential sensitivity to superfusion with the Ca2+-channel blocker cobalt (2 mM) or with low Ca2+ (0.25 mM). These procedures resulted in an AHP of much shorter duration (330 ms, range 150-600), presumably the FAHP. These observations indicate that a Ca2+-activated K+-conductance is likely to be involved in the generation of the SAHP. The FAHP was depressed during superfusion with tetraethylammonium (TEA) (20 mM) and intracellular cesium injection. The SAHP was enhanced by TEA and enhanced or depressed by cesium. In 3.6 mM K+ the FAHP reversed in polarity at membrane voltages more negative than -90 mV. This component had an approximately linear relation of amplitude to membrane potential. The SAHP did not reverse in most cells. In the few cases in which it reversed, the change in amplitude for a given change in membrane voltage was much smaller on the negative than on the positive side of the null potential. Thus the SAHP shows voltage-dependent, nonlinear characteristics. This difference in behavior of the two components was also observed when the null point was displaced in high or low K+. In the presence of tetrodotoxin (TTX) the AHP persisted in temporal association with a high-threshold, TTX-resistant, cobalt-sensitive spike. During the time course of the AHP the efficacy of synaptic input decreased, suggesting that the AHP has an important role in regulating the firing rate of the SPN.