1. In rat hippocampal slices, the lowering of the calcium concentration (from 2 to 1 mM) or diltiazem (200 microM) or CdCl2 (50 microM) significantly (P less than 0.01) increased the amplitude and shifted the stimulus-response curve of the secondary population spikes, due to paired pulse CA1 stimulation to the left. 2. The amplitude of the primary population spikes was not significantly affected. 3. The data demonstrated that the role of the concentration of calcium ions is critical in control of local inhibitory hippocampal circuitry at the base of paired pulse inhibition.