Attenuation of the neophobic rejection of novel flavours is mediated by a memorial process the properties of which were examined by testing its resistance to functional disruption. Rats maintained on a 24-h water deprivation schedule consumed only 3 ml of the novel apple juice but doubled its intake during the second presentation 2 days later. This attenuation of neophobia (AN) was prevented by pentobarbital anaesthesia (40 mg/kg) induced 0 and 1 h, but not 4 and 7 h, after the first apple juice presentation. AN was also disrupted by ether anaesthesia (5 min), hypothermia (20 degrees C body temperature), bilateral cortical spreading depression and electroconvulsive shock (50 mA, 1.5 sec) applied 0-1 h, but not 2 or more hours, after the first apple juice presentation. The results are consonant with the 'learned safety' hypothesis and indicate that a 2-4 h long continuous absence of noxious consequences of food ingestion is required to classify a gustatory stimulus as safe or neutral. Since interventions interfering with AN do not disrupt formation of conditioned taste aversion, the two forms of adaptive control of food selection are obviously mediated by fundamentally different neural mechanisms.