In order to evaluate the amount of Azidothymidine (AZT) and its metabolite (GAZT) in human plasma, the HPLC retention times of both compounds on a C18 column as well as on an anionic column by using, as eluent, the same mobile phase have been studied. The two chromatographic systems did not allow, separately, a simultaneous determination of AZT and GAZT. In fact AZT is easily revealed by a C18 column whereas the GAZT elution results too fast and the peak falls into plasmatic matrix. The opposite situation, namely the fastest AZT elution, has been observed by using the anionic column. In contrast by using the two columns linked together (max. back-pressure 21-22 MPa) both peaks were delayed conveniently and did not overlap with the plasmatic matrix. The total analysis time was about 11 min. long. The methodology reported here was further proved rapid and reliable as assessed by preliminary assays in plasma of an AZT treated patient.