The effect of serotonin (5-HT) on consolidation of a brightness discrimination reaction was investigated in rats. Five microgram 5-HT, injected intrahippocampally immediately after training, impaired retention of the brightness discrimination tested 24 hr later. In biochemical experiments, leucine incorporation into hippocampal proteins in vivo was 32% inhibited by 5 microgram 5-HT. Leucine incorporation into proteins of hippocampal slices in vitro was decreased by 5 x 10(-5)M 5-HT. The results seem to support Essman's assumption that inhibition of brain protein synthesis by 5-HT may be responsible for the memory impairment. But also some other possibilities for a mechanism of 5-HT amnesia must be discussed.