The dietary technique of acute tryptophan depletion was used to suppress brain serotonin synthesis in 16 women with documented premenstrual syndrome. Each subject was tested at distinct phases of each of two menstrual cycles. Baseline amino acid levels did not vary across the menstrual cycle, except for tyrosine which showed a significant premenstrual decrement. Compared to a sham procedure, actual tryptophan depletion caused a significant aggravation of premenstrual symptoms, particularly irritability. Symptom magnitude was correlated with diminution of tryptophan relative to other amino acids. This result supports other evidence implicating serotonin in premenstrual syndrome.