Doxorubicin (Adriamycin [AM]) was measured in plasma in the early distributive phase in 26 patients with different solid tumors. Pharmacokinetic parameters were calculated during first and/or subsequent courses of AM injected iv at doses from 40 to 60 mg/m2 either alone or with other antineoplastic agents. A fluorimetric technique was employed to determine AM equivalents; the reduced metabolite Adriamycinol was quantitated by scanning fluorescence after separation in thin-layer chromatography. The following conclusions are drawn from the findings. (a) After repeated AM treatments the individual variability in pharmacokinetic parameters of the drug did not appear to be affected by the length of treatment. (b) When the same patient was given different AM doses over a certain period some degree of dose dependence was seen only in the area under the curve and extrapolated plasma drug concentration at Time 0. (c) A narrow range of AM doses gave pharmacokinetic values differing from one patient to another. (d) After repeated AM treatments, Adriamycinol levels became lower, suggesting either a reduced metabolism of the parent compound or increased excretion of the reduced metabolite.