After oral administration of [3H]-labelled norethindrone-oxime (NETO) or norethindrone (NET) to rabbits, the blood concentrations of NETO and NET were determined by measuring the radioactivity after separation with HPLC. The pharmacokinetic parameters of the compounds were also compared. Experimental results indicate that both NETO and NET were quickly absorbed. Their elimination from blood revealed a fast and a slow phase. One portion of NETO was metabolized to NET, and the remainder was excreted in unchanged form. The amount of NETO and its metabolite, NET, in serum were about equal within 24 hours. The serum concentration-time curves of NETO and NET adequately fitted to a two-compartment model. There was significant difference between NETO and NET in Tmax (P less than 0.05). But no significant differences in other parameters (P greater than 0.05) were observed.