In pharmacokinetic/pharmacodynamic (PK/PD) modeling the first blood sample is usually taken 1 to 2 min after drug administration (late sampling). Therefore, investigators have to extrapolate the plasma concentration to Time 0. Extrapolation, however, erroneously assumes instantaneous and complete mixing of drug in the central volume of distribution. We investigated whether plasma concentrations obtained from early arterial blood sampling would improve PK/PD modeling. In 14 pigs, one of five neuromuscular blocking agents (NMBAs) was administered into the right ventricle within 1 sec and arterial sampling was performed every 1.2 sec (1st min). The response of the tibialis muscle was measured mechanomyographically. The influence of inclusion of data from early arterial sampling on PK/PD modeling was determined. Furthermore, the concentrations in the effect compartment at 50% block (EC50) derived from modeling were compared to the measured concentration in plasma during a steady state 50% block. A very high peak in arterial plasma concentration was seen within 20 sec after administration of the NMBA. Extensive modeling revealed that plasma concentrations obtained from early arterial blood sampling improve PK/PD modeling. Independent of the type of modeling, the EC50 and KeO based on data sets that include early arterial blood sampling were, for all five NMBAs, significantly higher and lower respectively, than those based on data sets obtained from late sampling. Early arterial sampling shows that the mixing of the NMBA in the central volume of distribution is incomplete. A parametric PD (sigmoid Emax) model could not describe the time course of effect of the NMBAs adequately.