Tyrosine, an aromatic amino acid which is the precursor of melanin and responsible for skin pigmentation, has been monitored in white and black skin of guineapigs after administration of psoralen, estradiol, testosterone propionate and hydrocortisone. Its level increases under the influence of the antivitiligo drug, psoralen, and decreases under that of hormones such as testosterone propionate and estradiol. Treatment with hydrocortisone shows a significant increase of tyrosine in white skin only. The implication of the results has been discussed in relation to melanogenesis in vitiligo.