Three groups of 24 male, and three groups of 24 female F344 rats were fed sodium nitrite for 2 years. Two male and two female groups received 2000 parts per million sodium nitrite mixed with powdered food. The remaining pair of groups received 2000 parts per million in drinking water at the rate of 100 ml per week. A similar pair of groups of untreated animals were maintained as controls. There was little difference in survival between the treated groups and the controls. The incidence of liver neoplasms, which were carcinomas and neoplastic nodules, in the two groups of females treated with sodium nitrite in feed was significantly higher than in the controls, but there was no significant difference in the males. The incidence of monocytic leukemia for each of the nitrite-treated groups was always lower than that for the matching untreated control groups, with the difference attaining or approaching significance in five of the six cases.