The objective of this study was to evaluate the chemical stability of 4-aminopyridine 5-mg capsules and 3,4-diaminopyridine 5-mg capsules under a variety of storage conditions. Each of the two drug preparations was extemporaneously prepared in hard gelatin capsules; lactose and micronized silica gel were used as excipients. Samples were stored under three conditions: refigeration at 4 deg C and protected from light for 6 months, protected from light at room temperature that ranged from 22 deg C to 24 deg C for 6 months, and at a temperature of 37 deg C and protected from light for 1 month. Once each month, visual inspection of the capsules and their powder contents was performed to identify observable changes (color, texture, etc) and the weight of the capsule content was measured individually. Chemical stability was assessed initially and at monthly intervals by means of a stability-indicating high-pressure liquid chromatography (HPLC) analytical technique based on the determination of drug content. No visible changes were observed in any of the samples under any of the storage conditions. The hard gelatin capsules remained clear and colorless, and the content of the capsules remained an off-white powder when viewed under normal fluorescent room light. Capsule content weight did not change during the study. Both 4-aminopyridine and 3,4-diaminopyridine exhibited excellent chemical stability under all study conditions. Little or no loss of drug content occurred in either product under refrigeration, at room temperature, and even at the elevated temperature of 37 deg C. The oral 5-mg capsules of 4-aminopyridine and 3,4 diaminopyridine did not undergo decomposition or other adverse changes within 6 months at refrigerated room temperature or within 1 month of storage at 37 deg C.