1. Stable intracellular recordings were obtained from neurons (n = 62) in the L6-S1 deep dorsal horn of the spinal cord in pentobarbital-sodium-anesthetized, intact rats (n = 26). All neurons responded to natural mechanical stimuli and/or electrical stimulation of peripheral afferents. 2. Intracellular penetrations were maintained for 30 min-2 h. Action potentials occurred spontaneously in most neurons (n = 50) and could be evoked in the remainder (n = 12) by depolarizing current passage. Mean resting membrane potential was -60.9 mV, mean action potential height amplitude was 75.2 mV, mean half-width of the action potentials was 0.33 ms, mean input resistance was 38 M omega, and mean time constant was 9.1 ms. 3. Action potentials were followed by afterpotentials made up of at least three components; a fast afterhyperpolarization (fAHP), a slow afterhyperpolarization (sAHP), and an afterdepolarization (ADP). Most neurons (n = 40) exhibited all three afterpotentials, although some displayed only a fAHP and an ADP (n = 10) or a fAHP and a sAHP (n = 12). The durations and magnitudes of the afterpotentials varied widely among neurons. 4. Steady-state current-voltage relations were investigated in 14 neurons with depolarizing and hyperpolarizing current pulses. Of these 14 neurons, 5 exhibited inward rectification, 3 had outward rectification, and the remaining 6 showed a predominantly linear change of membrane potential to current injection. In addition, several neurons (n = 9) exhibited a postinhibitory rebound that was sometimes (n = 4) accompanied by a "sag" in voltage during the preceding hyperpolarizing current step. 5. Four patterns of spike frequency adaptation occurred during step depolarizing current passage. The firing of most neurons gradually decreased with a simple, approximately exponential time course (n = 21), in some neurons it decreased with both a fast and a slow time course (n = 8), in several it incremented in rate (n = 3), and one neuron showed a complex combination of multiple decrementing and incrementing adaptations. Time constants, magnitude of adaptation, and the slopes of the steady-state current-voltage relation varied widely. 6. Oscillations in membrane potential and firing rate occurred in three neurons. The oscillations arose from endogenous mechanisms in at least one neuron because manipulation of membrane potentials altered the frequency of oscillation; a depolarizing current increased the period of oscillation and eventually produced tonic firing, and a hyperpolarizing current increased the frequency of oscillation and eventually terminated firing. 7. The results demonstrate that neurons in the L6-S1 region of the dorsal horn exhibit a diversity of cellular mechanisms that may significantly modulate normal somatosensory and visceral input.