1. The effects of stimulating the substantia nigra (SN) or the ventral tegmental area (VTA) on the excitability of cells in the rat hippocampal formation have been investigated in vivo. 2. A train of conditioning stimuli to either of the midbrain nuclei produced inhibition of evoked population spikes recorded in the CA1 pyramidal cell layer of the hippocampus. 3. These trains of pulses had no effect on the evoked synaptic field potential recorded in the stratum radiatum although they were effective in suppressing glutamate-induced firing of cells in the hippocampus. These observations suggest that the inhibition is mediated through a postsynaptic mechanism. 4. The inhibition of the test population spike was observed at a latency of 50 ms after the conditioning train to either the SN or the VTA but did not reach a maximum until 300-500 ms and 500-750 ms post-conditioning, respectively. The total duration of the inhibition in each case was about 5 s. 5. Following stimulation of the VTA, comparable levels of inhibition were recorded in the ventral and dorsal hippocampus. However, after stimulation of the SN, significantly less inhibition was observed in the ventral hippocampus. 6. Unlike the effects on the commissural-evoked population spike in CA1, stimulation of SN or VTA had no effect on perforant path-induced granule cell excitability in the dentate gyrus. 7. These results suggest that activity in the substantia nigra or ventral tegmental areas could have a powerful regulatory or feedback role in suppressing hippocampal excitability.