Male inbred 10-12 week old Wistar/Furth rats received either no carcinogen, or 1-2-dimethylhydrazine (DMH) 20 mg/kg body weight s.c. once weekly for 16 weeks, or N-[4-(5-nitro-2-furyl)-2-thiazolyl]-formamide (FANFT) 0.2% of the feed for 16 weeks, or DMH and FANFT concurrently. Thirty-three weeks after carcinogen exposure, all surviving treated and control animals were killed and examined for bowel and urinary bladder tumors. Adenocarcinomas of the large and small bowel occurred in approximately 33% of DMH-treated animals, and transitional cell carcinomas of the urinary bladder in approximately 33% of the FANFT-treated animals. After concurrent exposure to both carcinogens, no increased incidence of bladder tumors was noted compared to FANFT treatment alone. However, the number of animals with one or more adenocarcinomas of the bowel (22/30 versus 17/50, p < 0.001), the mean number of tumors per animal (2.1 +/- 0.2 versus 1.1 +/- 0.1, p < 0.01), and the invasiveness of the tumors through the bowel wall were all significantly increased after DMH + FANFT compared to DMH exposure alone.