An elevated plus-maze was used to study the ontogeny of the exploratory behaviors of earlyprotein-malnourished animals in an experimental aversive situation. Rats were malnourished by feeding their dams a 6% protein diet during the lactation period (0-21 days of age); controls received a 16% protein diet. From 21 to 100 days of age, all rats received a balanced lab chow diet. At 21,30, 40, 50, 60, 70, 80, 90, and 100 days of age, independent groups of malnourished and control animals were exposed to the elevated plus-maze and allowed to explore for 5 min. During this period, 12 behavioral categories (number of entries and time spent in the open and enclosed arms, total arms entries, attempts to enter open and closed arms, closed-arm returns, latency of fIrst closed-arm entry, total immobility time, total grooming time, total reatings, total head-dippings, and total stretch attend posture) were recorded. The results showed that anxiety increased with age and that protein malnutrition produced alterations in the ontogeny of exploratory behaviors. It was shown that malnourished rats maintained or increased exploration in the maze with age, in contrast to wellnourished animals, which exhibited a steady decrease in exploration as they matured. The higher number of entries and time spent in the open arms suggest that the protein malnutrition causes a permanent increase in the impulsiveness and/or an increased drive to explore open-arm novelty, leading to a behavior consistent with lower anxiety in malnourished animals, relative to that in wellnourished animals, in this naturalistic animal model of anxiety. Protein malnutrition early in life causes long-lasting structural and neurochemical changes in the central nervous system, as well as behavioral abnormalities (Dob-