We determined the blood plasma and tissue concentrations of the main fluorescent component of free Ukrain after a single intravenous injection of this substance (0.25 mg/kg) to intact and carcinosarcoma W-256 bearing rats using ion-pair high-performance liquid chromatography with fluorescent detection. The plasma and tissue concentrations of Ukrain were lower than expected, assuming its uniform tissue distribution and first-order kinetics. W-256 tumor tissue was the only tissue that showed accumulation of Ukrain (its concentration at 45 min after injection was up to 2.84-fold higher than in the blood plasma). Based on the elevation of Ukrain concentration in tissues at approximately 15 min after injection and taking into account its high molecular weight as well as the high dimensions of the Ukrain molecule or ion at physiological pH values, we conclude that the preparation penetrates into tissues by active transport or by favored diffusion. Due to the existence of such transport systems together with excretion of Ukrain and probably also its biotransformation, the disappearance of the preparation from the blood cannot be described using a one-compartment model. A relatively higher affinity for Ukrain was observed in tumor tissue and liver, while affinity was lowest in the brain and muscles. The presence of tumors decreased Ukrain concentrations in plasma and normal tissues in comparison with those in control animals.