A technique is described for extraction and quantitation of meperidine and normeperidine from breast milk by gas chromatography/mass spectrometry. Both compounds were measured in breast milk from parturients who received the drug for postpartum pain relief. The technique is sensitive to 1 ng ml-1 and standard curves were linear at 1.5-2000 ng ml-1. Levels in breast milk samples were 36.2-314 ng ml-1 and 0-333 ng ml-1 for meperidine and normeperidine, respectively. Preliminary results suggest that postpartum pain medication with meperidine results in considerable levels of both meperidine and its active metabolite, normeperidine, in breast milk.