The pharmacokinetics of pirarubicin and its active metabolite, doxorubicin, were studied after intravenous administration of pirarubicin (25-45 mg/m2) to ten pediatric patients. The concentration-time curves of pirarubicin in both blood and plasma showed representative biphasic patterns. Pirarubicin concentrations decreased rapidly from 0.5 to 2 h after administration and then decreased slowly until 24 h in all subjects. High concentrations of the metabolite, doxorubicin, were detected at 0.5 h after administration of pirarubicin which decreased gradually until 24 h. The area under the concentration-time curve from 0 to 24 h (AUC0-24) of pirarubicin and doxorubicin in blood were 3-4 times higher than those in plasma, suggesting that these drugs had a high affinity for blood cells. The AUC0-24 ratio of doxorubicin to pirarubicin in plasma was calculated to be 0.441. It might be indicated that not only pirarubicin but also doxorubicin are responsible for the therapeutic efficacy and the incidence of toxicity of pirarubicin. The pharmacokinetics of pirarubicin in pediatric patients was fundamentally similar to that of adults, but it was recognized that considerable interindividual variation in the disposition of pirarubicin and doxorubicin exists.