1. Early undernutrition can cause permanent functional changes in the central nervous system. Alterations in enzymes involved in neurotransmitter metabolism have been reported to result from early undernutrition. 2. In a previous study, we demonstrated that undernutrition during suckling decreases ATP and ADP hydrolysis by synaptosomes from cerebral cortex by about 20% of the value found in 20-day-old well-nourished rats (JBT Rocha, CF Mello, JJF Sarkis and RD Dias, British Journal of Nutrition, 63: 273-283, 1990). In the present study, we investigated whether this deficit persists in synaptosomes from cerebral cortex of nutritionally rehabilitated adult rats. 3. Rats were undernourished from birth to 25 days of life by feeding their dams a 7% casein (w/w) diet, while well-nourished offspring were fed by mothers maintained on a 28% casein diet. 4. In contrast to the results previously obtained in young rats, the synaptosomes obtained from the cerebral cortex of early undernourished adult rats hydrolyzed ATP and ADP more efficiently than did those obtained from well-nourished rats. Specific activity (nmol min-1 mg protein-1, mean+/-SD) was 114.9+/-9.5 for undernourished rats (N= 8) vs 94.1+/-9.5 for well-nourished rats (N= 8) for ATP, and 50.4+/-6.1 (N= 8) vs 38.8+/-4.5 (N= 8) for ADP. These results suggest that the deficits found in young rats disappear in rehabilitated adult rats.