Little is known about the psychobehavioral consequences of a dietary deficiency of the amino acid, L-lysine. This report demonstrates that a 4-d long L-lysine deficiency in rats interfered with the normal circadian release of the neurotransmitter serotonin, but not dopamine, measured by in vivo microdialysis in the central nucleus of the amygdala. L-Lysine deficiency was induced by feeding rats a L-lysine-deficient diet. Controls were pair-fed a L-lysine-sufficient diet. Footshock stress-induced anxiety, measured in an elevated plus-maze paradigm, and wrap-restraint stress-stimulated fecal excretion were significantly greater in the L-lysine-deficient rats than in the controls. We conclude that a severe deficiency of dietary L-lysine enhances serotonin release in the amygdala, with subsequent changes in psychobehavioral responses to stress.