Ethiofencarb and pirimicarb, widely used insecticides, were investigated for hepatocarcinogenesis-modifying effects using a medium-term bioassay system for carcinogens. F344 male rats were given a single intraperitoneal injection of diethylnitrosamine (DEN, 200 mg/kg) and then, starting 2 wk later, received ethiofencarb in the diet at concentrations of 500 or 250 ppm, pirimicarb at 400 or 200 ppm, or a combination of ethiofencarb (250 ppm) and pirimicarb (200 ppm) for 6 wk. Control groups received DEN (200 mg/kg) or carbamate insecticides alone as stated earlier, without DEN. All rats were subjected to two-thirds partial hepatectomy at wk 3 and were killed at wk 8. Development of preneoplastic lesions, glutathione S-transferase placental form-positive foci, in the liver was significantly increased in terms of number receiving 500 ppm ethiofencarb. The results thus indicate that ethiofencarb at high dose possesses promoting activity for rat liver carcinogenesis.