1. Simultaneous recordings were made from inhibitory cells located close to the stratum pyramidale and from pyramidal cells in the CA3 region of guinea-pig hippocampal slices, to examine inhibitory synaptic interactions. 2. The average amplitude of inhibitory postsynaptic potentials (IPSPs) initiated by single action potentials at different synapses varied between 0.3 and 2.6 mV. Experiments were performed to investigate the source of this variation. 3. Unitary IPSPs reversed at similar potentials to the first phase of the synaptic inhibition elicited by afferent fibre stimulation. IPSPs evoked by single action potentials or repetitive inhibitory cell firing were suppressed by picrotoxin, a gamma-aminobutyric acid (GABAA) receptor antagonists. 4. The time to peak and amplitude of averaged IPSPs were not related as predicted if amplitude variations resulted simply from different electrotonic locations of inhibitory terminals. 5. Transmission failures could be resolved at connections which generated small averaged IPSPs, but were not apparent at connections where averaged IPSPs were large. 6. IPSPs elicited by the same inhibitory cell in several pyramidal cells were of similar amplitude. The amplitudes of simultaneous IPSPs impinging on pairs of neighboring pyramidal cells were positively correlated. 7. Thus, the variation in efficacy of inhibitory synapses may result from differences in transmitter release from different inhibitory cells and not from postsynaptic factors.