Histological types and grades of 613 urinary bladder cancers induced in rats by N-butyl-N-(4-hydroxybutyl) nitrosamine(BBN) were analyzed. Most of them (95.1%) were transitional cell carcinomas, the remainder being squamous cell carcinomas (3.3%), undifferentiated carcinomas (2.5%), and carcinosarcomas (0.3%). Among the transitional cell carcinomas, 23.3% were Grade I anaplasia, 55.2% Grade II, and 21.5% Grade III. Among the squamous cell carcinomas, 20% each were Grade I and II, and 60% Grade III. Most of the undifferentiated carcinomas were Grade III. Some of the cases of transitional cell carcinoma had areas of squamous metaplasia and/or glandular metaplasia, and the incidence of metaplasia increased with the grade of anaplasia. Approximately 36% of the cases of transitional cell carcinoma were of the invasive type. Invasive types were twice as numerous among cases of squamous cell carcinoma as among those of transitional cell carcinoma and all of undifferentiated carcinomas were invasive. The incidence of invasive type was closely related to the grade of anaplasia. These results show that the morphological characteristics of urinary bladder carcinomas induced by BBN in rats are similar to those seen clinically in humans.