Little is known how synaptically released endogenous ACh affects hippocampal synaptic plasticity in vivo. Here, we examined the role of cholinergic drive in the regulation of the induction of long-term potentiation (LTP) at basal dendrites in the CA1 area of the anaesthetized rat hippocampus. The non-subtype selective muscarinic acetylcholine receptor antagonist, scopolamine, (0.3 mg/kg, i.p.) inhibited the induction of LTP by weak, but not strong, high frequency conditioning stimulation. A relatively M1 subtype-selective receptor antagonist, pirenzepine, (50 nmol/5 microL, i.c.v.) also inhibited LTP induction by the weak protocol. As the medial septum (MS) is a major source of endogenous ACh in the hippocampus, we also examined the effect of high frequency pre-conditioning stimulation of the MS on LTP induction. The pre-conditioning MS tetanus reduced the threshold for LTP induction at basal synapses in a narrow time window. Such an effect of MS pre-conditioning was prevented by scopolamine, strong evidence of a direct MS control of LTP threshold through a mechanism dependent on muscarinic receptor activation. These results suggest that the cholinergic drive to the hippocampus is critically involved in the control of the LTP induction threshold in vivo. To the extent that LTP mechanisms may underlie certain types of learning and memory, the septo-hippocampal cholinergic regulation of synaptic plasticity may constitute an important target for the treatment of cognitive disorders associated with ACh deficits.