Arecoline, a major alkaloid present in betel nut, was administered daily by gavage feeding to Swiss male and female mice at a dose of 1 mg/day/mouse five times a week, either alone or in combination with KNO3 or KNO3 + lime. Swiss mice of both sexes kept on a vitamin B complex-deficient diet were tested in a similar manner and compared with those receiving a normal diet. In the mice receiving a normal diet it was observed that arecoline induced tumors in 40% of males but failed to produce tumors in any of the females. Arecoline tumorigenicity in females was evident only when they received a vitamin B-deficient diet. Arecoline tumorigenicity was not evident in males when they were treated simultaneously with KNO3 + lime and kept on a normal diet. However, the same treatment administered to male mice kept on a vitamin B complex-deficient diet induced tumors in 39%.