The characteristics of glycinergic inhibitory postsynaptic currents (IPSCs) in sympathetic preganglionic neurons (SPNs) of neonatal rats were studied by whole-cell recordings in transverse spinal cord slices. In relation to postnatal age, the decay time constants of these currents decreased without a comparable effect on their rise time. This may result from alpha-subunit switching of the glycine receptor and/or increased glycine uptake during this period of postnatal life. The kinetics of glycinergic IPSCs were also temperature- and voltage-dependent. Whereas, compared with room temperature, rise and decay of the events were faster at more physiological temperature, only the decay increased upon depolarization. Visual identification of SPNs was confirmed by intracellular staining and comparison with retrogradely labeled SPNs.