Plasma concentration-time profiles of meperidine after intravenous administration to 7 pregnant women during labor were investigated. There was considerable interpatient variation in initial plasma concentrations of meperidine and its volume of distribution. When the data in the literature on the disposition of meperidine in nonpregnant subjects were used for comparison, it was found that the initial dilution volume (V1), the volume of distribution at steady-state (Vss), and the apparent volume of distribution during the terminal exponential phase of drug elimination (Vbeta) were less (p less than 0.05) in pregnant subjects than in healthy nonpregnant control subjects but were not significantly different in pregnant subjects and nonpregnant surgical patients. Mean total systemic blood clearance was also significantly less in the pregnant subjects (843 ml/min) than previously reported for healthy nonpregnant control subjects (1,465 ml/min). The blood/plasma concentration ratio of meperidine in pregnant women (0.940) was higher than that in control subjects (0.768; p less than 0.01). Placental transfer of meperidine was investigated in 4 of the patients; in 2 the concentration of the drug was greater in cord plasma than in maternal plasma. Further study is indicated.