effects of malnutrition and environment on the acquisition and extinction of avoidance behavior in rats twelve newborn rats were fed by mothers maintained on a proteindeficient diet 12 casein m during lactation and 12 rats fed by mothers maintained on a diet containing 25 casein were used as controls c after weaning all animals were fed standard lab ration half of each group was housed individually mi and ci while the other half was allowed to live in pairs mp and cp when adult all animals were trained to avoid footshocks by jumping onto a platform training sessions consisted of 40 trials starting with a 20 sec light stimulus cs and followed by a 2 sec 06 ma shock us with an average intertrial interval of 54 sec when all animals displayed consistent avoidance behavior the extinction phase was initiated the procedure was the same as for the training sessions except that the shock generator was disconnected extinction continued until each animal showed a 50 reduction in avoidance performance during acquisition mi learned faster than ci and ci showed greater avoidance performance than cp but no differences were observed between mp and cp during extinction group m responded more persistently than group c the present acquisition results may explain the contradictory data reported in the literature with respect to the effects of malnutrition on avoidance performance since environmental stimulation was shown to reduce the effects of early malnutrition individually housed animals showed greater avoidance performance during both phases