Although previous research with adult humans and nonhumans has suggested a relationship between sleep behavior and brain serotonin levels, no studies have been made of the relationship of normal children's or infants' sleep patterns to serotonin levels, tryptophan metabolism, or diet. This study investigates the relationship between dietary influences on behavioral organization in the human newborn after feeding and synthesis of the brain neurotransmitter serotonin. A total of 20 healthy full-term infants, appropriate for gestational age, were assigned to either tryptophan or valine feeding conditions. The tryptophan treatment group received a feed containing 33 mg/dl of tryptophan dissolved in a 10% glucose solution to maximize the carrier transport of tryptophan across the brain/blood barrier; the valine group received 50 mg/dl of valine, dissolved in a 5 percent glucose solution, to minimize tryptophan transport. Each infant was observed continuously for 3 hours after feeding, and records were made of the infants' state of consciousness at 1-minute intervals. Results suggested that modifying infant feeding preparations with tryptophan and carbohydrate mixtures produces an effect on newborn state behavior without disrupting such state characteristics as mode of onset through active sleep. Variations in serotonin levels in newborn brains thus appear to play a role in modulating the newborn's arousal behavior. (RH)