Fluctuations in the availability to the brain of tryptophan and tyrosine cause major changes in the rates at which neurons synthesize serotonin and the catecholamines respectively. Such changes occur when the pattern of neutral amino acids in the plasma is altered by what has been eaten, the plasma changes inducing parallel changes in the amounts of tryptophan or tyrosine that are transported from blood to brain. Diet-induced changes in plasma choline levels can produce similar changes in acetylcholine synthesis. The three nutrients, tryptophan, tyrosine and choline, when administered in the pure form or simply ingested in food, can thus act like drugs giving rise to important changes in the chemical composition of structures in the brain. The interactions that relate the amount of a nutrient administered or ingested to its level in the blood plasma, its level in the brain and its effect on nerve neurotransmission, are not simple. The conversion of tryptophan into serotonin is influenced by the proportion of carbohydrate in the diet; the synthesis of serotonin in turn affects the proportion of carbohydrate an individual subsequently chooses to eat. In the case of choline and tyrosine the effect on a neuron of an increased supply of the nutrient varies with the neuron's firing frequency and can lead to changes in that frequency. Choline and tyrosine can hence amplify neurotransmission selectively, increasing it at some synapses but not at others. Taken together, these findings illustrate a novel and hitherto unsuspected aspect of nutrition's effects on the brain and provide the basis for new modes of therapy for patients with some metabolic, neurologic or psychiatric brain diseases.