Recent studies indicate that British anti-lewisite (BAL) administration may reduce dermal inflammation due to sulfur mustard exposure. To better understand the relationship between the efficacy of BAL and plasma concentrations, the pharmacokinetics of the compound need to be examined and characterized. In these studies, BAL was administered intramuscularly (im) to hairless guinea pigs (HGPs) at 50 ug/kg and 25 ug/kg (n=8). Following BAL administration, serial blood samples were obtained at 0, 1, 5, 10, 15, 30, 60, 120, 240, and 360 min. The resulting plasma samples were assayed for unbound BAL and the internal standard ethane dithiol using a novel gas chromatographic/mass spectrometric (GC/MS) method. The analytical method involved the liberation/extraction of the free drug from plasma, followed by preparation of the pentafluoropropionyl derivative by reaction with 1-pentafluoropropionylimidazole. The mean plasma concentration-time data were fit to a standard one-compartment pharmacokinetic model. Pharmacokinetic parameter estimates determined by the model were maximal plasma concentrations (Cmax) of 16.3 and 32.8 ng/ml, time to maximal plasma concentration (Tmax) of 1.8 and 2.9 min, apparent volume of distribution (Vd) of 1.5 and 1.4 l/kg, area under the time-concentration curves (AUC) of 851 and 1598 ng·min/ml, and half-life of elimination (T1/2-elim) of 34.9 and 31.7 min for 25 and 50 ug/ml, respectively. These data suggest that this method is suitable for examining the pharmacokinetics of BAL in HGP plasma samples.