Tryptophan and nonesterified fatty acids (NEFA) are bound to serum albumin. NEFA displace tryptophan from their binding sites. Since NEFA decrease during phototherapy, we examined reciprocal variations of NEFA, total and free tryptophan in jaundiced newborn infants during phototherapy. After 24 h of phototherapy, we noticed a significant decrease of both NEFA and free tryptophan (p less than 0.001). Free tryptophan variations might affect the synthesis of cerebral neutrotransmitter 5-hydroxytryptamine, which depends on the passing of free tryptophan through the blood-brain barrier.