The acute cytogenetic effects of tyramine and MTCAs, precursors of the mutagen present in soy sauce, were studied with the in vivo chromosome aberration test in rat bone marrow cells. The chemicals were administered intraperitoneally. Statistically significant positive results were obtained with tyramine at a dose of 5 mmole/kg (686 mg/kg) body weight and with MTCAs at doses over 0.50 mmole/kg (115 mg/kg) body weight, respectively. Chromosome aberrations (CA) induced by L-proline co-administered with either tyramine or MTCAs were significantly lower than those induced by each chemical alone. These data suggest that L-proline, after endogenous nitrosation, became nitrosoproline and suppressed CA, and that, as a result of in vivo nitrosation of tyramine and MTCAs, they became mutagenic nitroso compounds showing positive results. Statistically significant positive results were obtained by administration of 40 mmole NaCl/kg body weight (2338 mg/kg). The cocarcinogenic role of NaCl with tyramine was suggested because soy sauce contains about 18% NaCl.