In an attempt to elucidate mechanisms involved in adaptation to a novel environment, consequences of neurotoxic damage induced by administration of monosodium glutamate (MSG) to both male (n = 42) and female (n = 45) rats in the early postnatal period were studied. Rats treated with MSG and appropriate controls were tested on postnatal days 21 and 65 for alterations of exploration and the rapidity of habituation changes in an open field test. Compared with intact animals, a high dose of MSG (4 mg/g) increased exploratory behaviour, with a subsequent decrease in the rapidity of habituation of male rats. Neonatal stress represented by hypertonic saline injection in a vehicle-control group induced a slight increase of exploratory behaviour as compared with intact animals. Males proved to be more vulnerable to neonatal MSG treatment and handling than females. These results suggest a negative effect of neonatal stress and treatment with MSG on habituation to a new environment in male rats.