1. A case of metol poisoning is described in which the patient manifested cyanosis and black urine.2. The phenomena are considered to be due chiefly to the presence of aromatic amino and hydroxyl groups in the original molecule.3. This molecular arrangement closely imitates the structures of substances formed in the blood in enterogenous methaemoglobinaemia.4. The presence of a hydroxyl group in the original drug is essential for the production of the fully developed syndrome.5. The chemistry and toxicology of coloured substances are reviewed in detail.It is a pleasure to acknowledge the help I received from my laboratory assistant, Mr.J.A.E. Biggs.