Figure 4. Total plasma meperidine concentrations were essentially constant (1.2 +/- 0.1 micro gram/ml) during a 5-h infusion, started at elapsed time zero. In contrast, normeperidine concentrations increased significantly from 0.04 +/- 0.01 micro gram/ml 1 h after starting administration to 0.28 +/- 0.13 micro gram/ml 5 h after starting meperidine administration. Oxygen consumption did not increase significantly in these normothermic volunteers, indicating that normeperidine per se does not induce muscle rigidity or shivering.