K203 is an experimental bis-pyridinium mono-aldoxime type cholinesterase reactivator of potential use in organophosphate/ organophosphonate poisoning. Pharmacokinetics of K203 were examined in Wistar rats and beagle dogs using ion-pair HPLC. Serum and cerebrospinal fluid concentrations of K203 were determined using ion-pair reversedphase chromatography on octadecyl silica column. HPLC with ultraviolet detection was used for determination of serum concentration of K203 higher than 0.1 μg/mL while its low concentrations in cerebrospinal fluid required electrochemical detection (0.015 through 4 μg/mL range). In rats the serum levels of K203 followed zero order pharmacokinetics from 15 to 120 minutes post administration. Zero order pharmacokinetics was also observed in beagle dogs after low dose (15 μmol/kg) of K203 administration. High dose administration (250 μmol/kg) led to subsequent hindered elimination from both cerebrospinal fluid and serum.