Sprague-Dawley rats bearing Yoshida Tumour showed an increased excretion of 7-methylguanine and 1-methylhypoxanthine with their urine as compared to control animals. The concentration of these methylated purines, which belonged to the non-specific cancer markers, was already significantly higher 2 days after tumour injection and more than 2-fold after 3 additional days. This was proved by daily L-[14CH3]methionine supply. Separation and identification of these compounds was done by liquid chromatography and mass spectrometry. By RNA analysis it became evident that tumour RNA contained more methyl groups than liver RNA of control rats. This suggested that the increased excretion of 7-methylguanine and 1-methylhypoxanthine should not be only ascribed to an increased turnover of tumour RNA but also to its higher degree of methylation.