Brain serotonin (5HT) metabolism in rats at weaning was examined in relation to their maternal dietary protein (PC) and tryptophan (TRP) concentration and to their protein feeding behavior. Pup brain TRP, 5HT, and 5-hydroxyindoleaceticacid (5HIAA) concentration was unaffected by maternal diet PC (10, 20, or 40%) but was increased by TRP additions (0.66 or 1.86%) to 40% casein diets. Turnover of brain 5HT, measured at weaning after probenecid injection, was inversely related to maternal diet PC (r = .54, P less than 0.05) but was not affected by TRP supplementation. Pups allowed to select from 10 and 60% casein diets selected during the first 3 days post-weaning, dietary PC which reflected the PC (r = 0.65, P less than 0.01), but not RRP content of their maternal diet, and also reflected brain 5HT turnover (r = .50, P less than 0.05), but not concentration, of their littermates. Thus, the relationship dietary PC to protein selection by the offspring is associated with maternal diet-maternal diet-induced changes in their brain 5HT turnover, at least as measured after probenecid administration.