Carcinogenic activity of several synthetic N-nitroso compounds was evaluated in C57BL/6J X C3HeB/FeJ F1 mice. Test substances, suspended in trioctanoin, were injected i.p. in three equal doses given on Days 1, 4, and 7 after birth and animals were held without further treatment for up to 85 weeks. Nitrosoephedrine at a total dose of 600 mg/kg induced metastasizing liver cell carcinomas in 28 of 30 animals. Nitrososarcosine (225 mg/kg) induced similar tumors in 8 of 14 animals. Nitrosofolic acid (375 mg/kg) induced lung adenocarcinomas in 4 of 28 mice. Creatinine-5-oxime (600 mg/kg) showed no evidence of carcinogenic activity. Diethylnitrosamine (12 mg/kg given in four doses), included as a positive control, caused metastasizing liver cell tumors in 23 of 25 animals.