Sensitivity to light was investigated 5 and 8 h after consumption of an amino acid drink which contained L-tryptophan (balanced amino acid condition: 19 controls and 22 migraine sufferers) or which produced a short-term reduction in brain serotonin synthesis by omitting L-tryptophan (tryptophan depletion condition: 16 controls and 16 migraine sufferers). Migraine sufferers reported more intense nausea, headache, glare- and light-induced pain than controls. In addition, glare- and light-induced pain were greater in the tryptophan depletion condition than in the balanced amino acid condition, in both migraine sufferers and controls. Eight hours after the amino acid drink, after participants had completed tests of pain sensitivity and motion sickness provocation, tryptophan depletion augmented headache in migraine sufferers and aggravated nausea in migraine sufferers and controls. These findings suggest that a reduction in brain synthesis of serotonin intensifies photophobia and other migrainous symptoms and thus might contribute to the pathogenesis of migraine.