PIP: The plasma metabolic clearance rate (MCR) and the urinary excretion of norethindrone were measured in 8 volunteer women, who took Norlutin (2.5 mg norethindrone per day for 20 days per month) for 6 to 7 months. MCR increased significantly (p.05) after 6 months of norethindrone medication. Urinary excretion of norethindrone and its metabolites did not change significantly after the 6 month course. Variability in the MCR measurements among the subjects could not be accounted for by size differences or different times of the menstrual cycle. Total plasma radioactivity (from tritiated norethindrone) fell much more slowly than plasma norethindrone levels; this is probably due to conjugated metabolites of norethindrone.