The stability of phenobarbital sodium diluted to 10 mg/mL in 0.9% sodium chloride injection was studied at an unadjusted pH (8.5) and an adjusted pH (10.0) at 4 degrees C over a four-week period. The phenobarbital concentrations were measured before dilution and at 1, 2, 3, 7, 14, and 28 days using high-performance liquid chromatography. The observed phenobarbital concentrations ranged from 98.0 to 106.0% of initial concentration at the unadjusted pH and from 96.7 to 108.5% of initial concentration at the adjusted pH. The diluted phenobarbital sodium was stable over a four-week period at 4 degrees C without the need for pH adjustment.