PIP: A quantitative procedure using deuterated carrier and gas chromatography-mass spectrometry is described and applied to quantitate norethindrone levels in milk from 5 lactating women, aged 20-30 years, who had received 1 silastic implant of 40 mg of norethindrone. The technique is a multiple ion one (gas chromatography-mass spectrometry). The deuterated norethindrone was used as the internal standard and carrier for the detection technique, and, after its addition, the milk samples were extracted with Amberlite XAD-2 columns and purified on Sephadex columns. O-methoxime-trimethylsilyl ether derivatives of the purified samples were analyzed. The assay was precise and reproducible. The mean level of norethindrone at 3 weeks post implant insertion was 208 pg/ml of milk, after which it declined slowly and steadily.