Cholinergic input to the hippocampus originates in the septum and diagonal band. Guinea pig hippocampal slices in a bath of carbachol, a cholinergic agonist, displayed different patterns of rhythmical activities depending on the carbachol concentration. Exposure to 50 or 100 microM led to intermittent induction of theta-like activities (TLAs). Long-term potentiation (LTP), induced by tetanus. was facilitated at concentrations within the optimum range for generating TLAs. This facilitation was blocked by the muscarinic receptor antagonist atropine. Augmentation of LTP during TLAs was greater than that during the rest period of TLAs which, in turn, was greater than that induced without activation of the muscarinic receptors. These results suggest that there are two muscarinic facilitation processes of LTP, one dependent on and the other independent of TLAs, with the former being more easily facilitated than the latter.