An ion-pair high pressure liquid chromatographic method is described for the determination of cytarabine (CAS 147-94-4, araC) in human plasma. Complete separation is achieved within 10 min using a reversed stationary phase and an isocratic eluent containing 0.4 mmol/l heptane sulfonic acid as modifier. Detection by UV-absorption occurs at 270 nm. Quantification of cytarabine and of its main plasma metabolite uracil arabinoside (araU) is achieved by means of internal standardisation using adenine arabinoside (araA). Retention times of araU, araC, and araA are 3.9, 5.9 and 9.4 min, respectively. Detection limits of araC and araU are 10 and 15 ng/ml, resp. During a pharmacokinetic study of high-dose cytarabine treatment no interferences could be observed in plasma samples.