Trimorphamide [N-(1-formamido-2,2,2-trichloroethyl)morpholine], with an acute LD50 of 990 mg/kg, is a systemic fungicide under development. To examine its possible nitrosation in vivo, CFY rats were treated i.g. with 326 mg/kg trimorphamide, suspended in 40 mg/kg sodium nitrite. after 1 hour, another 40 mg/kg sodium nitrite was given i.g. and the animals were killed an hour later. The nitroso derivative was extracted by dichloromethane from the gastric content, after steam distillation. The samples were analysed by gas chromatography (GC) and mass spectrometry (MS). N-Nitrosomorpholine (NMOR) was identified by comparison with authentic samples, both by GC and MS, using retention times and electron impact mass spectra. Quantification was carried out by GC-MS-specific ion monitoring, using the m/e 116 peak. The yield of NMOR ws found to be 0.65%. By the use of the Ames test, NMOR formed in vivo proved to be strongly mutagenic for the strains TA 1535 and TA 100, both in presence and absence of S-9 fraction. Tumour-bearing animals, treated by sodium nitrite plus trimorphamide, are being analysed.