Carboplatinum was injected into four groups of mice for 5 successive days at one out of four daily equidistant hours. Upon the cessation of these injections, granulocyte colony stimulating factor was injected into half of these mice for 2 days at the same times. The stimulating factor was also injected into groups of untreated mice for 2 days at the same times. In each experiment the rhythms of four variables were monitored after the cessation of all injections: peripheral WBC count, bone marrow and sperm stem cells DNA synthesis, and sperm ribosomal RNA content. The results indicated that in the course of repeated drug administrations, variables' rhythm parameters (phases and periods) were altered. These changes have to be considered in chronotherapeutic schedules when variables which serve for toxicity diagnosis or as drug target are rhythmic in nature.