A novel form of long-term potentiation of glutamatergic synaptic transmission is described in the rat medial habenula nucleus. It occurs when uridine 5′-triphosphate is bath applied at low micromolar concentrations and is prevented by Reactive Blue 2, suggesting that it is mediated by P2Y4 receptors. Uridine 5′-diphosphate can also cause such a Reactive Blue 2-sensitive potentiation, but at higher concentrations (200 µm), suggesting that this might also be an effect on the relatively uridine 5′-diphosphate-insensitive P2Y4 receptor. The potentiation is due to an increase in presynaptic release probability. It requires neither depolarization nor calcium influx postsynaptically and is thus probably non-Hebbian. When potentiation due to low concentrations of uridine 5′-triphosphate is inhibited in the presence of Reactive Blue 2, uridine 5′-triphosphate causes instead a significant inhibition of glutamate release. We suggest that this inhibition may be mediated by a Reactive Blue 2-insensitive P2Y2-like receptor. At higher concentrations of uridine 5′-triphosphate (200 µm), the inhibitory effect dominates such that even in the absence of Reactive Blue 2 no potentiation is seen.