The influence of one single intraperitoneal application of 50 mumol/kg fenfluramine (FF) or 50 mumol/kg parachloroamphetamine (PCA), respectively, on the 8th postnatal day upon avoidance learning in the early ontogenesis was compared with vehicle-injected controls (NaCl solution). The intoxicated groups were unable to learn a self-shaped avoidance behaviour in a peripheral field avoidance test. They did not find out during the third postnatal week that only central fields of an open field remained unpunished; escape responses along walls were not inhibited in favour of escape into central fields as in controls. The passive avoidance component which is expressed by increasing stay duration in central fields was not developed in FF and PCA rats. The active avoidance component expressed by CS-induced escape before UCS did also not increase in the intoxicated rats. There were no differences of passive avoidance between groups, however, in the fourth week. In the W-maze test after weaning differences between intoxicated groups and controls were evident in the first session and when the schedule was changed. In the reversal learning session the intoxicated groups made more errors. The results support the view that serotoninergic neuron groups participate in the inhibition of incorrect responses and differentiation learning. This early postnatal deficiency may be partly compensated in further development.