Albino rats received 10 s of sub-seizure electrical stimulation applied to the dentate gyrus granule cells immediately after acquisition of information on trial 1 of a 2-trial radial maze spatial memory task. Granule cell stimulation selectively reduced the probability of accessing information held in declarative memory ('knowing that' a particular maze location contains food) while leaving procedural memory intact ('knowing how' to search for food in the maze). This specific memory impairment was prevented by pretreatment with the opiate antagonist naloxone. Naloxone also improved memory performance when given to non-stimulated subjects.