In the spleen, thymus and liver of mice and rats carrying transplantable rapidly growing hepatomas a shift between the de novo and "salvage" pathways of the pyrimidine RNA nucleotides synthesis in favour of the de novo one was observed. The phenomenon was found to be the manifestation of a drastically diminished uridine utilization by the host tissues, this precursor being preferentially and avidly used by the tumour for its own RNA synthesis. The above findings are regarded as the result of a successful competition of the tumour with the host tissues for vital metabolites representing one of the forms of the systemic effects of malignant neoplasms.