Intracellular recording techniques have been used to study neurons that respond to low- and to high-intensity mechanical stimulation of the skin of the hindpaw (wide dynamic range or multireceptive cells) in the deep dorsal horn of the fourth lumbar segment of the spinal cord, in decerebrate-spinal rats. Electrical stimulation of the A-fibers in the sciatic nerve produced a short-latency response in all 32 neurons studied. A long-latency prolonged excitation was produced in 28 of the 32 neurons when the unmyelinated afferents in the sciatic nerve were activated. This paper describes the physiological properties of 12 multireceptive cells with A- and C-fiber inputs, whose cell body location was established by horseradish peroxidase ionophoresis and the morphology of six neurons in this group whose cell bodies lay within lamina V. Single stimuli to the sciatic nerve at an intensity high enough to activate unmyelinated afferent fibers (C-fiber strength) produced two patterns of response in the neurons. In five neurons a number of long-latency postsynaptic potentials (PSPs) clearly separated from the short-latency A-fiber evoked PSPs were produced, resulting in an early discharge, a silent period, and a late discharge. The second pattern, found in seven neurons, was a long-lasting depolarization, only generated by C-strength stimuli, which continued from the early A-fiber evoked PSPs, peaked at 100-200 ms, and lasted for 300-500 ms, producing in six cases a continuous burst of action potentials with a maximal frequency at the expected latency of the C-afferent fiber input but with no clear A- and C-fiber evoked banding of the action potentials. This postsynaptic depolarization was large enough to inactivate action potentials in one cell. Repeated stimuli to the sciatic nerve (1 Hz for 10 s) at C-fiber strength produced five different types of response in the neurons. In three neurons a progressive increase in the size and duration of the C-fiber PSPs occurred, resulting in an increase in the number of action potentials (windup), whereas in two, the repeated stimulation resulted in a progressive moderate depolarization of the neurons and an increase in the total number of action potentials evoked at both early and late latencies. Large depolarizations, sufficient to partially inactivate action potentials, developed during the repeated stimulation in two cells, effectively reducing the number of spikes evoked per stimulus.(ABSTRACT TRUNCATED AT 400 WORDS)