4-Chloronitrosopiperidine was administered to a group of 15 male Sprague-Dawley rats as a 0.005% solution in drinking water for 27 weeks. Of the 15 animals, 12 died with tumors induced by the treatment, half of them by the 41 st week; all 15 were dead at 58 weeks. Seven rats had liver tumors, 5 had esophageal tumors, 2 had tumors of the nonglandular stomach and 2 had tumors of the nasal turbinates. Several rats had tumors of more than one organ.