The divalent cation zinc is known to modulate chloride currents carried by native and recombinant mammalian glycine receptors (GlyRs). To unravel the effect of Zn2+ on glycinergic neurotransmission, inhibitory postsynaptic currents (IPSC) of rat spinal neurons grown in culture were analysed in the absence and presence of Zn2+. Low concentrations of Zn2+ (0.5 and 5 micro m) augmented the mean amplitude of miniature IPSCs by approximately 40% over values obtained in the absence of zinc, whereas higher concentrations of Zn2+ (50 micro m) significantly decreased mean amplitude values. Remarkably, low concentrations of Zn2+ also significantly increased the mean frequency of miniature IPSCs. This effect was blocked by the P2X receptor antagonists PPADS and suramin, indicating the presence of Zn2+-sensitive presynaptic P2X receptors on glycinergic terminals. Immunostaining with antibodies against different P2X receptor subtypes revealed that P2X2 receptors partially colocalize with the GlyR. Potentiating concentrations of Zn2+ also affected the kinetics of miniature and evoked IPSCs by significantly prolonging their decay time constants. Electrophysiological analysis of heterologously expressed glycine transporters (GlyT) revealed for GlyT2 zero, and for GlyT1 a modest (< 20%), reduction of glycine uptake in the presence of 5 micro m Zn2+, indicating that prolongation of glycinergic IPSCs by Zn2+ is not due to inhibition of glycine removal from the synaptic cleft. Together, these results suggest that Zn2+ is a potent modulator of glycinergic synaptic transmission which increases in a synergistic manner the agonist affinity of both presynaptic P2X2 receptors and postsynaptic GlyRs.