The efficacy of s.c. administration of 1-methyl-1-nitrosourea (MNU) for the induction of mammary carcinomas was compared with the i.v. method of carcinogen injection in female Sprague-Dawley rats. Group-housed animals fed a laboratory chow diet and distilled water ad libitum throughout the study were injected at 50 days of age with 50 mg MNU per kg body weight. The carcinogen was given either s.c. or i.v., via the jugular vein, to one of the two groups of 20 rats each. Animals were palpated for tumor detection weekly and necropsied 180 days after injection with the carcinogen. At the termination of the study, 180 days postcarcinogen, cancer incidences were similar, 95 versus 90% in animals given MNU either s.c. or i.v. with an average of 3.9 and 3.9 cancers per rat, respectively. Time of tumor appearances were essentially identical under both treatment conditions. Using either method of carcinogen administration resulted in the induction of approximately 2.4 times more carcinomas in the cervical-thoracic mammary glands than in the abdominal-inguinal glands with no differences observed in cancer occurrence in the left versus the right mammary gland chains. The data indicate that s.c. administration of MNU is as effective and specific in the induction of mammary carcinomas as is i.v. administration. The s.c. method has the advantage of being easier and faster to perform and permits reproducible treatment of large numbers of rats by a small technical staff.