The consequence of early overnutrition on later behaviour of rats was tested during the first year of life under conditions of different complexity. The behaviour patterns of overfed rats were compared with those of animals, kept under common nutritional conditions and of rats undernourished prior to weaning. It was demonstrated, that early overnutrition is not beneficial for later exploratory behaviour, learning ability, and the resistance to the pharmacological or nutritional stress. The trend of the observed behavioural responses was identical both in the over- and undernourished rats, differences appeared to be quantitative rather than qualitative.