Carcinogenicity of phenacetin was tested using Sprague-Dawley rats. Two groups of animals containing 50 males and 50 females per group were fed, respectively, with 2.5% and 1.25% phenacetin diet for 18 mo. and thereafter with basal diet for 6 mo. Control animals (65 males and 65 females) were fed with basal diet for 24 mo. Animals surviving more than 24 mo. were regarded as effective animals and killed. Rats that died of tumor development within 24 mo. were also regarded as effective animals. Every organ from the killed and dead animals was fixed in 10% formaldehyde solution and examined histopathologically. Effective number of rats was 27 males and 27 females in 2.5% phenacetin feeding group, and 22 males and 25 females in 1.25% phenacetin feeding group. In the control group, 19 males and 25 females were effective. Neoplasms including spontaneous tumors were detected in 26 of 27 males (96.3%) and 21 of 27 females (77.8%) of the 2.5% phenacetin feeding group, and in 20 of 22 males (90.9%) and 19 of 25 females (76.0%) of the 1.25% phenacetin feeding group. In the control group, 1 of 19 males (5.3%) and 6 of 25 females (24.0%) showed spontaneous tumor development. Histopathologically, carcinomas of the nasal cavity, such as adenocarcinoma, squamous cell carcinoma and transitional cell carcinoma; and the urinary passage, such as renal cell carcinoma of the kidney pelvis and transitional cell carcinoma of the urinary bladder were most conspicuous, suggesting the target organs of phenacetin carcinogenesis. Males showed higher tumor incidence compared to females. The higher the concentration of phenacetin given, the higher incidence of tumors observed.