5-Chloro-2'-deoxycytidine (NSC 371331, CDC) is in development as a possible radiosensitizing agent for cancer treatment. Previous studies have been done to demonstrate the in vivo efficacy of CDC with various modulators of its metabolism. This paper describes our preclinical studies to determine the pharmacokinetic properties of CDC and the disposition of the drug, both alone and in the presence of the metabolic modulator tetrahydrouridine (THU), a cytidine deaminase inhibitor. Detection of the drug in biological fluids was performed by HPLC analysis using a C-18 column, gradient elution with solvents composed of aqueous trifluoroacetic acid and acetonitrile, and ultraviolet absorbance at 290 nm. Samples were processed by treatment with ammonium sulfate prior to injection into the HPLC system. CDC was stable in aqueous solution and in mouse plasma. High doses of CDC (100mg/kg) were given i.v. or i.p. to mice for the determination of CDC plasma half-life (10 min). CDC was not detectable in plasma after oral administration. It was converted rapidly to 5-chloro-2'-deoxyuridine (CDU) by cytidine deaminase, and CDU was readily discernable in plasma and urine samples collected after i.v. and i.p. administration of CDC. When CDC in doses ranging from 5 to 100mg/kg was given with 100mg/kg of THU, increased plasma levels of CDC were seen. CDC was eliminated through the kidneys, as well as by enzymatic deamination, and did not bind to plasma proteins. The initial steps of the CDC metabolic pathway were determined in vitro with isolated enzymes. Cytidine deaminase from mouse kidney converted CDC into CDU; thymidine phosphorylase converted CDU into 5-chlorouracil (5-CU). The conclusions of these studies are: (a) CDC is a drug with a short half-life and (b) it is excreted through the kidney, mainly in metabolite form. Administration of THU substantially increased the concentrations of CDC in mouse plasma, supporting proposals that the combination of THU with CDC should be evaluated in clinical trials.