The compatibility and stability of mephentermine sulfate with hydrocortisone sodium succinate or aminophylline in 5% dextrose injection were studied. Mephentermine sulfate injection (equivalent to mephentermine 300 mg) was added to both hydrocortisone sodium succinate injection (equivalent to hydrocortisone 100 mg--admixture I) and aminophylline injection 250 mg (admixture II), each in 5% dextrose injection 400 ml. The solutions were stored in sealed USP type I containers and stored at 0-4 degrees C, 30 +/- 2 degrees C, and 45 +/- 1 degree C. Aliquots were drawn initially and at 4 and 24 hours for pH, drug content, and degradation product determinations. Mephentermine sulfate content was analyzed by a spectrophotometric method involving ion-pair extraction with tropaeolin 00. Hydrocortisone sodium succinate and aminophylline were analyzed by color reaction with isoniazid and UV spectrophotometry, respectively. The content of mephentermine and total hydrocortisone showed no change in admixture I under all test conditions. Hydrocortisone sodium succinate ester hydrolyzed slowly, but not more than 5% after 24 hours at 45 degrees C. There was no appreciable loss of mephentermine sulfate or aminophylline potency in admixture II. The pH of the solutions did not change over the period of study. The results indicate that both admixtures were compatible and stable under the tested storage conditions for at least 24 hours.