1. Pharmacokinetic properties of a new derivative of the widely used and very potent antileukemic agent arabinosylcytosine (araC)--5'chloro-5'-deoxy-arabinosylcytosine (5'-Cl-araC)--were investigated after intraperitoneal (i.p.) and oral routes of administration in rats and compared with the equimolar dose of araC administered orally. 2. It was found that substitution of the hydroxyl group at position 5' resulted in a change of pharmacokinetic parameters. 3. There is a large difference in average serum concentrations of 5'-Cl-araC administered by the i.p. and oral routes; the average serum concentration obtained after i.p. injection being several times higher in comparison to those after oral administration. 4. However, the latter are, at the same time, lower than the average serum concentrations of araC administered by the same route in an equimolar dose. 5. On the other hand, the apparent volume of distribution is much larger, and the area under the curve of serum concentration of 5'-Cl-araC is smaller, after oral as compared to the i.p. route of administration indicating more extensive tissue distribution together with higher tissue binding of 5'-Cl-araC when compared to the parental drug araC.