Cytosine arabinoside (ara-C) has been used in the treatment of leukemia, but its exact mechanism of cytotoxicity is not yet known. One of the proposed mechanisms for the effectiveness of this drug in treating leukemias suggests that a metabolite of ara-C, i.e., 2'-deoxycytidine 5'-triphosphate (araCTP), competes with cytosine arabinoside 5'-triphosphate (dCTP) for binding to DNA polymerase. The ratio of the drug metabolite to the endogenous nucleotide (araCTP/dCTP) may, therefore, be important in determining the effectiveness of ara-C therapy. This ratio may also play a role in drug resistance. Previously published methods have focused on either araCTP or dCTP, along with metabolites and analogues of one of these compounds. The methods presented here provide two simple, sensitive ways to measure dCTP and araCTP in the same biological sample.