RESUMEN
A specific radioimmunoassay was developed for the estimation of norethindrone levels in the serum of lactating women. A conjugate of norethindrone-3-BSA was synthesized and antiserum raised against it in rabbits. The antiserum showed high affinity (1.5 x 10(9)M/L) and titer and was specific. The sensitivity of the assay was 125 pg/ml serum. Serum levels of norethindrone were estimated in 25 lactating women, inserted with subdermal implant releasing about 150 ug of norethindrone acetate daily. The norethindrone levels were initially high in the first two months. From there onwards the levels of norethindrone remained constant with an average value of 1.0 ng/ml up to six months. These values were comparable to those obtained in non-lactating women. Thus, lactation does not seem to alter or affect the release of norethindrone acetate from the subdermal implant or its metabolism.