A combined antitumor effect of a single injection of doxorubicin and the indole hormone melatonin was investigated in the model of transplantable Ehrlich carcinoma in female SHR mice. Animals received melatonin either subcutaneous 10 mg/ kg in the evening or with water 10 mg/l at night (from 20 pm to 8 am) for 3 weeks. The results of an experiment showed that combined use of doxorubicin and melatonin both subcutaneous and with water leads to statistically significant inhibition of tumor growth compared with control and doxorubicin alone.