Reversed-phase separation of various pyridinium aldoximes requires a certain concentration of ion-pairing agent, as their chemical structures contain two quaternary amines in the pyridinium ring. Adequate mobile phase is scouted on the basis of retention of pyridinium aldoxime (using the graph of k' versus concentration of an ion-pairing agent) compared to the chromatogram of the background peaks originated from the homogenate. Change in the ion-pairing agent concentration was more expressed for the elution of K-203 than that of the background peaks from the serum, brain and cerebrospinal fluid. Stability of K-203 was investigated using HPLC. Determination of K-203 in tissue samples requires homogenization using either trichloroacetic acid or perchloric acid. Fast degradation takes place at acidic pH. Adjusting pH to neutral in the possible shortest time frame helps to avoid degradation. Degradation of K-203 was easily followed by HPLC separation and monitoring the elution with an ultraviolet absorbance detector at 276 nm. Amperometric detection indicates only the decrease of K-203 content.