We investigated the relationship between the effects of cholecystokinin (CCK) and kainic acid (KA) in the CA3 region of hippocampal slices from rats. As has been reported previously, KA in nanomolar concentrations caused spontaneous epileptiform discharges (bursts) and an excitatory shift of the input/output (I/O) curve. CCK octapeptide (100-200 nM) applied alone had no effect on spontaneous activity or I/O curves. Pretreatment of slices with sulfated CCK blocked the effect of KA on synaptic transmission, but had no effect on KA-induced bursting. Pretreatment with nonsulfated CCK had no effect.