An HPLC method for the determination of in vivo release of norethindrone-alpha, beta-poly (3-hydroxypropyl)-DL-asparamide (NET-PHPA) conjugate was established using column switching technique. The sample prepurification could be omitted. It was extracted by a precolumn (ODS, 9-11 microns) and the analytical column was packed with ODS (15 cm x 4 mm ID. Shimpac CLC). The mobile phases were water and methanol--water (7:3) respectively. The experiment showed that the recovery from serum was 82.6%, the RSD within day was 1.3-1.8% and that in day-to-day was 0.7-5.6% (n = 4).