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Spontaneous locomotor activity, exploratory behavior, pain reactivity and swimming ability were tested in 18-, 24-, 36-, and 60 days old pups from mothers which had been nourished on a protein-methionine low diet (PMLD) (7,8% protein, 0.1% of a daily requirement of methionine) for two months before conception throughout gestation and weaning; afterwards the young were fed on this diet until the age of 60 days and were compared with the offspring of rats, whose mothers and they themselves were nourished throughout hat period on a standard diet (STD). The motility of 24 and 36 days old pups receiving the PMLD was significantly higher and than of 60 days old lower than in controls. Analogous results were obtained in ambulation in the open field test. There were no differences in peeping and rearing in comparison with controls throughout the experiment, and only after 60 days both behavioral responses were significantly decreased. From the 24th day of life the PMLD rats revealed a statistically significant decrease in grooming and defecation frequency. There were no differences in swimming. The reactivity to a pain stimuli (hot-plate test) was higher than in controls on the 24th day of life. Generally, after an increase in activity on the first days of postnatal life a decrease in exploratory and emotional activity was observed after the 60th day of life.