Abstract: A method to determine the concentration of Ara-a in plasma by reversed phase high performance liquid chromatography is established. The method offers the advantage of rapid andimple preparation of plasma samples and sampling volumes as small as 0. 5 ml. There is no need to ex-tract Ara-A (adenine arabinoside) or to remove or modify sugar inoities to obtain samples of adequateseparation. In pharmacokinetic studies, when amounts of clinical material are small and large numbersof assay are required, the present method is very useful. The calibration curve is linear for a widerange of concentration. The recovery is 87.5%. After a single dose of Ara-A-NC adenine arabinoside nanocapsules or Ara-A injection via intra-venous administration in rabbits, the Ara-A concentration in plasma was determined by this method.The pharmaceutical parameters of Ara-A-NC and Ara-A injections were obtained by using PKBP-N1program in NEC computer. The data obtained fitted an open two-compartment model well with twocorrelation coefficients more than 0. 99. In mice given Ara-A injection a shorter elimination half-time (20. 32 min), MRT (9. 647 min) and a smaller AUC (233. 4 μg·min·ml^-1) were obtained comparedwith the group given Ara-A-NC injection. For the latter, the figures were 67. 82 min, 156. 7 minand 309.9 μg·min·ml^-1, respectively. It can be concluded that NC can alter the pharmacokinetic beh-avior of Ara-A in mice significantly and keep the drug at a higher level and for a longer time.