The cholinergic system plays a crucial role in learning and memory. Modulatory mechanisms of this system in the acquisition and consolidation processes have been extensively studied, but their participation in the memory retrieval process is still poorly understood. Conventional pharmacological agents are not highly selective for particular muscarinic acetylcholine receptor subtypes. Muscarinic toxins (MTs) that are highly selective for muscarinic receptors were extracted from the venom of the mamba snake, like the toxin MT3, selective for the M4 receptor subtype. These toxins are useful tools in studies of the specific functions of the M4 mediated transmission. The M4 receptor selective antagonist MT3, given into the dorsal hippocampus before the test, have enhanced the memory retrieval of an inhibitory avoidance task in rats. MT3 had no effect in the habituation to a new environment, including basic motor parameters, meaning that the effect in he inhibitory avoidance is purely cognitive. Our results suggest an endogenous negative modulation of the cholinergic muscarinic system upon the retrieval of previously consolidated aversive memories, hereby shown by the facilitatory effect of MT3.