1. By using the whole-cell recording configuration of the patch-clamp technique in a spinal cord slice preparation, we have made recordings from visually identified neurones in the lateral horn of the thoracic and lumbar spinal cord of neonatal rats (newborn to 14 days postnatal). 2. Some of the recorded neurones were labelled with the fluorescent dye Lucifer Yellow (n = 27). Their morphology was typical for sympathetic preganglionic neurones (SPNs). Based on the size of the cell soma and the electrophysiological properties, unlabelled neurones were also regarded as SPNs. 3. Spontaneous synaptic activity of different patterns could be observed in 73% of the recorded neurones (n = 106). It reversed at the chloride equilibrium potential (ECl) and could be reversibly blocked by strychnine (1-10 microM), but not by bicuculline (10 microM) or SR95531 (5-10 microM). 4. Synaptic activity could be elicited by focal electrical stimulation in the vicinity of the recorded neurone. These evoked synaptic events exhibited features similar to the spontaneous synaptic activity. 5. Application of glycine (100 microM-1 mM) by a fast microperfusion system induced a chloride current in twenty-seven out of thirty cells tested. The currents were reversibly blocked by strychnine (1-10 microM), but were only weakly sensitive to bicuculline (10 microM). Stability of current responses to glycine was increased by inclusion of ATP (4 mM) in the intracellular medium. 6. Application of gamma-aminobutyric acid (GABA; 100 microM-1 mM) by the fast microperfusion system induced a chloride current in all twenty neurones tested. These currents were reversibly blocked by bicuculline (10 microM). Strychnine (1-10 microM) blocked this current only weakly. Run-down of GABA-induced currents was prevented to a great extent by inclusion of ATP (4 mM) in the pipette. 7. These results suggest that the inhibitory synaptic activity recorded from SPNs in thin, transverse slices of neonatal rat spinal cord is mediated by glycine receptor-gated Cl- channels. GABAA receptor-gated Cl- channels might be activated by inputs from other spinal segments and/or descending pathways from higher brain regions.