OBJECTIVE To research if the improvement in psychomotor development observed during the treatment of malnutrition, is related to favorable changes in the speed of nerve conduction and in the excretion of hydroxy-indole acetic acid (indole). DESIGNED OF THE STUDY: Prospective, of a descriptive type, includes the follow-up of children during the first month of treatment. CONCLUSIONS To better know the impact which malnutrition has on mental development. METHODS Nine children, ranging from three to 15 months of age, gravely undernourished. METHODS The neurological development, the speed of nerve conduction and the excretion of indole were evaluated at 10 day intervals. RESULTS Using the Gesell technique in order to evaluate the development, the measurement of conduction through the medial and external popliteal sciatic nerves and indole excreted in 24 hours, a quantitative increase of all of these variables was seen in 24 hours. At the beginning the conduction speed was slow (less than 30 m/s) and the excretion of indole was very low (0.28 mg/24 h). After the tenth day these measurements returned to normal, although the development coefficient was found to still below at the end of the study (63.9 +/- 21.0). CONCLUSIONS Simultaneous to the neurological deficit, there is a reduced speed of conduction, which returns to normal after the tenth day without a positive correlation with motor functions and development. Neither does the disponibility of serotonin (judged by the excretion of indole) correlate with the speed of conduction.