Surprisingly little is known about muscarinic regulation of synaptic plasticity in the CA3 region of the hippocampus. Williams and Johnston (1988, 1990) demonstrated a muscarinic receptor-mediated depression of mossy fiber (MF)-LTP through a presynaptic site of action and Maeda et al. (1993) proposed a bidirectional modulation of MF-LTP by muscarinic receptor subtypes. Since then, this issue has remained largely neglected, not the least because of the lack of ligands endowed with a high degree of selectivity for the different muscarinic receptor subtypes. Here, we performed field potential and whole-cell recordings from the hippocampal CA3 region of M₂ receptor single knockout and M₁/M₃ receptor double knockout mice to determine the role of these receptors in short-term and long-term plasticity at associational/commissural (A/C) and MF inputs to CA3 pyramidal cells. At the A/C-synapse, M₂ receptors tonically inhibited basic excitatory transmission, but promoted short-term facilitation and LTP, whereas M₁/M₃ receptors did not affect these properties. M₂ receptors mediated the opposite effect on LTP at the MF synapse, which was significantly reduced. Notably, M₁/M₃ receptors suppressed MF-LTP in a synergistic fashion. Our data provide a first picture of how cholinergic projections to area CA3 recruit different muscarinic receptor subtypes to control synaptic plasticity in a highly coordinated, reciprocal and input-specific manner. Supported by the DFG (AL 294/9-1)