Abstract 1.1. Aims: This is a case report that exemplifies the chronopharmacokinetic differences of Doxorubicin (Dox), depending on time of administration of the drug because of biological rhythms, and its repercussions on adverse reactions. 1.2. Case Report: A 9-year-old female patient with Osteosarcoma (OS), with two pharmacokinetic studies of DOX conducted, in two different days, received two 48 hours-long intravenous infusions of this drug. There was a two-week period in-between, maintaining the same length of administration, and only differing by the time of day the first starting at 16 hours and the second at 2 hours am. 1.3. Results: There were no adverse effects in the patient at the end of the first infusion. However, after 8 hours of completing the second infusion with DOX, the patient presented a state of shock, low blood pressure and symptoms including extreme fatigue, disorientation, nausea, diarrhea, and general pain. The pharmacokinetic profiles obtained at the end of each infusion revealed that during the second infusion, the patient had a greater area under the curve (1.626 ng/h /ml vs 1.391 ng /h/ ml), with a much shorter half-life time (2.19 hours vs. 6.63 hours t1/2). 1.4. Conclusions: Our report suggests that these adverse effects may be related to the time of drug was administrated by effects of biological rhythms.