Half the dogs exposed to an aerosol of virulent canine distemper (CD) virus died. Dogs which died did not develop antibody, whereas survivors developed antibody by the 9th day. Certain tissues were examined using fluorescent antibody. On the day after exposure, virus was demonstrated only in bronchial lymph nodes and in tonsils; it appeared in the blood on the 2nd and 3rd days, but only in mononuclear cells. From the 6th to 9th days, mononuclear cells carrying virus were seen throughout the body, and dogs without antibody had widespread invasion of all epithelial tissues. After epithelial invasion, signs of clinical illness were seen, sometimes characterized by anorexia, diarrhoea, conjunctivitis, and terminal convulsions. Virus disappeared in dogs, which developed antibody and survived, except in a few dogs which had virus in neurones and in epidermal cells of foot pads. This may account for certain c.n.s. syndromes and hardened foot pads often seen in dogs after their apparent recovery from the disease.