The effects of bath-applied sulphated cholecystokinin octapeptide (CCK-8S; 500 nM) and the selective CCKB receptor antagonist PD 135158 (1 microM) on the induction and maintenance of an 'unsaturated' long-term potentiation (LTP) were examined using recordings of field excitatory postsynaptic potentials (fEPSP) and population spikes (PS) in the hippocampal CA1 region of the rat. LTP was induced by a single theta-pulse stimulation (10 pulses at 7 Hz) of the Schaffer collateral-commissural pathway. CCK-8S had minor effects on the induction of LTP but substantially prolonged LTP of both fEPSP and PS from about 1 to at least 3 h. The antagonist PD 135158 had no influence on the induction of potentiation of fEPSP slope, but decreased the potentiation of PS amplitude. The results support the view that CCK-8S may facilitate long-term changes in glutamatergic synaptic transmission of the hippocampus, such as LTP.