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The traditional approach to pharmacokinetic/pharmacodynamic modeling of muscle relaxants requires sampling of plasma to determine drug concentrations. The authors recently proposed that certain pharmacodynamic characteristics (IR50, the steady-state infusion rate to maintain 50% twitch depression; keo, the rate constant for equilibration between plasma concentration and effect; and gamma, the Hill factor describing sigmoidicity of the concentration-effect relation) could be estimated without plasma concentration data. Here estimates for IR50, keo, and gamma determined with and without plasma concentration data are compared.Six volunteers were given 15–60 micro gram/kg vecuronium on each of two occasions during anesthesia with propofol. Mechanical responses to train-of-four stimulation were measured at the adductor pollicis and at the laryngeal adductors. Various pharmacokinetic models accounting for the presence and potency of vecuronium's 3-desacetyl metabolite and a sigmoid e-max pharmacodynamic model were fit to the resulting plasma concentration and effect (adductor pollicis and laryngeal adductors) data to determine IR50 keo, and gamma for each effect. One model related dose to effect without plasma concentration data.Values for IR50 (adductor pollicis), IR50 (laryngeal adductors), gamma (adductor pollicis), and gamma (laryngeal adductors) were similar when determined with and without plasma concentration values. Values for keo (adductor pollicis) and keo (laryngeal adductors) were larger when determined without plasma concentration values compared with those determined with these values; however, the ratio of keo (adductor pollicis) to keo (laryngeal adductors) was similar when determined with and without plasma concentration values.Certain pharmacodynamic parameters were estimated accurately in the absence of plasma concentration values. This suggests limited utility for plasma concentration data under conditions similar to those of the present study.