The posttraining IP administration of adrenaline (epinephrine) HCl (5.0 micrograms/kg) or tyramine HCl (1.0 mg/kg) causes retrograde amnesia for a one-way step-down inhibitory avoidance task in rats. The effect is cancelled by the simultaneous injection of the alpha 2-adrenergic receptor antagonist yohimbine HCl (2.0 mg/kg) or of the beta 1 - beta 2 blocker propranolol HCl (2.0 mg/kg). The amnestic effect of posttraining adrenaline or tyramine is counteracted by the administration of adrenaline or tyramine prior to testing: each drug has a greater antiamnestic effect against itself than against the other drug. The antiamnestic effect of tyramine and adrenaline is antagonized by the simultaneous administration of prazosin or yohimbine, but not by that of propranolol. We conclude that the posttraining amnestic effect of adrenaline and tyramine is mediated by alpha 2 receptors (probably postsynaptic) and that it does not reflect a storage deficit, since memory can be restored by an appropriate treatment given before the test session. The antiamnestic effect of adrenaline and tyramine is mediated both by alpha 1 and by alpha 2 receptors, and probably reflects the dependency of mechanisms that make stored information available for retrieval on circulating catecholamines. The present findings provide no clue as to the anatomical distribution of the adrenergic receptors involved in the amnestic or antiamnestic actions of adrenaline and tyramine.