The stability of aminophylline injection in crystalline amino acid-dextrose solutions containing standard additions of electrolytes, vitamins, and minerals was evaluated. Aminophylline injection was added to three parenteral nutrition (PN) solutions in dosages of 0.25-1.50 mg/ml. All of the samples were prepared in duplicate, stored in sealed volumetric flasks, and allowed to stand under normal lighting at room temperature or under refrigeration for 48 hours. Samples were analyzed for theophylline content by reverse phase high-pressure liquid chromatography at 1, 24, and 48 hours. Mean percent theophylline recovery 24 hours after admixture was 101 +/- 10.9%, 101 +/- 4.3%, and 100 +/- 4.3% in the three PN solutions. Solution pH values were stable for 48 hours. Refrigeration and lower amino acid concentrations did not alter stability. Aminophylline is stable in PN solutions for 24 hours in concentrations up to 1.5 mg/ml.