The intraperitoneal application of L-nucleosides (L-Cyd, L-Urd, L-dThd) to mice results in distribution of these compounds into tissues of the organism and their gradual excretion in the unchanged form. The residual level has been observed with L-ribonucleosides only and contains in addition to the L-nucleoside its 5'-phosphate. The phosphorylation in vivo is catalyzed by nucleoside-kinase and utilizes ATP as the phosphate donor while glycerol 1-phosphate and creatine phosphate are inactive. The L-cytidine derivatives are in vivo deaminated to the derivatives of L-uridine. On the other hand, when L-uridine is applied in vivo, derivatives of L-cytidine are obtained on the level of both the nucleoside and 5'-ribonucleotide.