Because of the unavailability of sensitive analytic techniques, the pharmacokinetics of meperidine have not been clearly delineated in obstetric patients during labor. Moreover, the production of the active meperidine metabolite--normeperidine--has not been investigated. By means of gas chromatographic and mass spectrometric techniques, these characteristics of meperidine metabolism were evaluated in 23 pregnant patients in the present study. The data show that the disappearance curve and pharmacokinetic constants for meperidine are similar to those previously reported for nonpregnant subjects. In regard to normeperidine, the data indicate that it is produced within ten minutes after meperidine injection, increases rapidly for the next 20 minutes, and then slowly increases throughout labor. The results enumerate the pharmacokinetic constants of meperidine in obstetric patients and describe the appearance of normeperidine, the active meperidine metabolite, following meperidine administration during labor.