The chemopreventive activity of two synthetic retinÃ¡rmelesof relatively low toxicity against A/-butyl-A/-(4-hydroxybutyl)nitrosamine (OH-BBN)-induced urinary bladder cancer was stud ied in F344 rats and C57BL/6 x DBA/2 F! mice. Female and male rats were given a total dose of either 1800 or 3200 mg OH-BBN over a period of 6 or 8 weeks, respectively. Male mice were given a total dose of either 90 or 180 mg OH-BBN over a period of 9 weeks. Seven days after the final intubation of OH-BBN, animals were fed either a placebo diet or a diet supplemented with the following retinoids: for rats, 0.8 mmol 13-c/s-retinoic acid, 2 mmol A/-(ethyl)-all-frans-retinamide, or 2 mmol A/-(2-hydroxyethyl)-all-frans-retinamide per kg diet; and for mice, either 0.5 or 1.0 mmol of A/-(ethyl)-all-frans-retinamide or /V-(2-hydroxyethyl)-all-frans-retinamide per kg diet. Animals were killed 6 months after the initial gastric intubation. In comparison to male and female rats fed placebo diets, all three retinoids reduced the incidence, number, and severity of the low-grade papillary transitional cell carcinomas of the urinary bladder. Similarly, treatment of mice with either of the two retinamides reduced the incidence of highly invasive urinary bladder carcinomas. The chemopreventive effect of the less toxic retinamides was equal to or greater than that of 13-c/sretinoic acid.