The stability of penicillin G potassium injection after reconstitution was evaluated in two different diluents of sodium chloride $0.9\%$ and dextrose $5\%$ in water stored at room temperature or refrigerated condition. The concentrations of penicillin G, stored for 24 hours at room temperature or for 10 days at refrigerated condition, were determined by HPLC. Also the pHs of the reconstituted solutions were monitored. The concentrations and pHs of penicillin G potassium 500,000 U/ml injection after reconstitution gradually decreased in all conditions. Stored at room temperature after reconstitution, a new peak which suspected as degradation products of penicillin G was detected in 5 hours in sodium chloride $0.9\%$, 4 hours in dextrose $5\%$ in water. At refrigerated condition, the new peak was detected in 4 days in both sodium chloride $0.9\%$ and dextrose $5\%$ in water. The degradation products of penicillin G allergy have been thought to be one of the substances responsible for evoking allergic reactions. In conclusion, the penicillin G potassium 500,000 U/ml injection after reconstitution was stable for 4 hours in sodium chloride $0.9\%$ 3 hours in dextrose $5\%$ in water solution at room temperature. At refrigerated condition, both solutions were stable for 3 days after reconstitution.