Nitrous acid (HNO2) was partly extracted from water by organic solvents, especially polar ones. Carbaryl was nitrosated in solvent: water mixtures most rapidly when nonpolar solvents, e.g. methylene chloride, carbon tetrachloride and hexane, were used. Under given conditions, carbaryl was nitrosated in methylene chloride:water mixtures 20 times faster than in water alone, mainly because of its insolubility in water. For ethylurea, hexylurea, ethyl N-ethyl-carbamate and aminopyrine, nitrosation by sodium sulfate-dried methylene chloride extracts of nitrous acid ('dried HNO2') was at least 88% complete after reaction for 5 seconds at 6 degrees C. Nitrosation of N-butylacetamide by the same extract proceeded more slowly, with a second-order rate constant 31,000 times greater than for nitrosation in water at pH 2. Butylacetamide was nitrosated in methylene chloride by equivalent concentrations of 'dried HNO2', dinitrogen trioxide and dinitrogen tetroxide at similar rates. Nitrosomethylurea (NMU) formation was measured radioactively in the stomach contents of rats fed [3H]methylurea (MU) and sodium nitrite. When both compounds were given in the food (100 mg MU and 4 g NaNO2/kg) and the rats were killed 3 hours later, the NMU yield was 0.46% of the MU. When the food also contained 11.5 g sodium ascorbate/kg, NMU production was completely inhibited. With 2-4 g/1 sodium nitrite in the drinking water and MU in the food, no NMU was detected. Ureas were determined in dried, salted bonito fish from Japan, by a method involving ion-exchange and paper chromatography. The fish sample contained 80 mg urea/kg, but no MU. When the fish was nitrosated at pH 1 and then denitrosated at pH 0, 25 mg MU/kg was detected. Mu identity was confirmed by mass spectrometry.