Post-training administration of the acetylcholine muscarinic M2 presynaptic receptor antagonist AF-DX 116 (0.1-10.0 mg/kg, ip), facilitated 48 h retention, in male Swiss mice, of a one-trial step-through inhibitory avoidance task. The dose-response curve was an inverted U. AF-DX 116 did not increase the retention latencies of mice that had not received a footshock during training. The influence of AF-DX 116 (1 mg/kg, ip) on retention was time-dependent, which suggests that the drug facilitated memory storage. The memory facilitation induced by AF-DX 116 (1 mg/kg, ip) was prevented by atropine (0.5 mg/kg, ip) administered after training, but 10 min prior to AF-DX 116 treatment. In contrast, neither methylatropine (0.5 mg/kg, ip), a peripherally acting muscarinic receptor blocker, nor mecamylamine (5 mg/kg, ip) or hexamethonium (5 mg/kg, ip), two cholinergic nicotinic receptor antagonists, prevented the effects of post-training AF-DX 116 on retention. Low subeffective doses of the central acting anticholinesterase physostigmine (35 micrograms/kg, ip), administered immediately after training, and AF-DX 116 (0.1 mg/kg, ip), given 10 min after training, acted synergistically to improve retention. The effects of AF-DX 116 (0.1 mg/kg, ip) were not influenced by the peripherally acting anticholinesterase neostigmine (35 micrograms/kg, ip). Considered together, these findings suggest that the activation of a muscarinic cholinergic presynaptic inhibitory mechanism, probably by increasing brain acetylcholine release, may modulate the activity of post-training processes involved in memory storage.