In order to examine whether the effects due to cytostatics might be improved by synchronization, cytosin-arabinoside, hydroxyurea or Vincristin were administered to rats bearing solid transplantation tumors, at different intervals following synchronization. The three substances, interfering with the S-phase of the cells, show significantly improved therapeutic results with quickly proliferating Walker-carcinosarcoma after a synchronization by hydroxyurea. Cyclophosphamide on the other hand, being effective through several phases of the cell cycle, furnishes no evidence of any essential therapeutic result. Analogous therapeutic experimentation using slowly growing neurosarcomas yielded no better results after synchronization. The concept of synchronization treatment may be approved, therefore, with certain limitations: firstly, synchronization with hydroxyurea can be maintained for a short time only, and the cancerous cells pass throught the first phases of the cell cycle following synchronization for the most part together with normal cells; secondly, only tumors with an important growth fraction and only cytostatics that do not affect several phases of the cell cycle are appropriate for synchronization therapy.