Single oral doses of 0.3 mg, 0.5 mg and 5 mg Norethindrone (NET) and 0.5 mg and 5 mg Lynestrenol (lyn) were given to five women. Lynestrenol is probably metabolized through NET and exerts its main biological activity as NET. Plasma concentrations of NET were determined by a radioimmunoassay at different intervals after administration of the tablets. Peak concentrations of NET were found within two hours after intake of each table. The plasma half life of Net after NET and lyn administration for the period 8-24 h was 8-11 h. No significant difference was found between the half life of NET and the NET tablets and after the lyn tablets. When 5 mg NET was given the plasma half life of NET for the period 24-72 h was around 10 h and this was significantly shorter than the half-life of NET after 5 mg lyn, which was 16 1/2 h. The systemic availability of the drugs was estimated by calculating and comparing the areas under the plasma concentration versus time curve (AUC). 0-24 H. The AUC 0.24 after 0.3 MG NET was almost identical to the AUC 0.24 after 0.5 mg lyn. The AUC 0-24 after 0.5 mg NET was significantly larger than after 0.5 lyn. No difference was found between the AUC 0-24 after 5 mg lyn and 5 mg NET. This study supports the concept of a conversion from lyn to NET. It also shows that there were only minor pharmacokinetic differences between the drugs when all samples were measured as NET.