1. We studied excitatory synaptic currents activated by stimulation of Schaffer collateral-commissural fibres and recorded from interneurones in the CA1 region of hippocampal slices using whole-cell techniques. 2. Interneurones were identified by their location outside the cell layer and their morphology as seen with differential interference contrast (DIC) microscopy and by filling with Lucifer Yellow (LY). 3. The excitatory postsynaptic current (EPSC) had a fast, voltage-insensitive component and a slow component which had a region of negative slope resistance between -70 and -40 mV. The slow voltage-dependent component was abolished by the N-methyl-D-aspartate (NMDA) receptor antagonist (DL-2-amino-5-phosphonovalerate (APV) 50 microM) which had little effect on the fast component. Conversely, the fast component was abolished by the non-NMDA receptor antagonist 6-cyano-7-nitoquinoxaline-2,3-dione (CNQX; 10 microM), which had no effect on the slow component. 4. The rise time of the fast component ranged from 1 to 3 ms and the decay time constant ranged from 3 to 15 ms. The rise time of the slow component ranged from 5 to 11 ms and the decay time constant ranged from 50 to 100 ms. 5. It is concluded that although the morphology of the excitatory synapses onto interneurones differs considerably from those onto pyramidal cells, their electrophysiological and pharmacological properties are very similar.