1. The actions of 5-hydroxytryptamine (5-HT) on rat dentate gyrus neurones were measured with conventional intracellular recording techniques in brain slices maintained in vitro at 32 degrees C. 2. Bath application of 5-HT (0.3-100 microM) hyperpolarized the membrane potential and reduced the input resistance; these effects persisted in tetrodotoxin (1 microM) and were abolished by MDL 73,005EF, a 5-HT1A receptor antagonist. 3. Local application of 5-HT via a pressure pipette also elicited a hyperpolarization and a reduction in resistance, and evoked a transient 'burst' of spontaneous inhibitory postsynaptic potentials (IPSPs) which were blocked by tetrodotoxin or bicuculline. 4. The 'burst' of IPSPs was subject to desensitization. It was completely abolished in the presence of the 5-HT3 receptor antagonist dolasetron. 5. In some cells, a longer lasting increase in spontaneous IPSP frequency was observed during application of 5-HT; this effect was blocked by the 5-HT2 receptor antagonist MDL 100,907. 6. 5-HT (30 microM) shortened the decay time constants of the glutamatergic and GABAergic evoked EPSPs and IPSPs without changing their amplitudes. 7. It is concluded that 5-HT hyperpolarizes granule cells via postsynaptic 5-HT1A receptors and increases spontaneous GABA release from inhibitory interneurones via the activation of 5-HT3 receptors and/or 5-HT2 receptors.