The activity of serotoninergic neurons of the medial raphé nucleus was turned off by local injection of the neurotoxin, 5,7-dihydroxytryptamine. Seven days later, a conditioned passive avoidance reaction was developed following 20 preexposures to the conditional stimulus (the presentation of the experimental chamber). As compared with the sham-operated control, in which 20 preexposures to the stimulus elicited latent inhibition, i.e., attenuation of the reproduction of the conditioned passive avoidance reaction, prolonged maintenance at the formed level, and lack of susceptibility to amnesia, all of the enumerated features were disturbed in animals administered the neurotoxin. The administration of haloperidol (0.5 mg/kg) 1 h prior to training reestablished the state of latent inhibition which had been disrupted by the switching off of the mesolimbic serotonin system. It is hypothesized that the process of the suppression of attention to the nonreinforced signals during the preexposure to the stimulus is achieved by the interaction of the serotoninergic and dopaminergic systems.