To determine the chemical compatibility of three different triple drug admixtures diluted with either 5% dextrose in water or 0.9% NaCl solution when administered via a multiple line infusion system (Omni-Flow 4000, Abbott Laboratories, Abbott Park, IL). The triple drug admixtures were: a) dobutamine, dopamine, and norepinephrine; b) nitroglycerin, sodium nitroprusside, and dobutamine; and c) nitroglycerin, dopamine, and dobutamine. Two phase in vitro compatibility study. Pharmaceutical laboratory. None. Phase I assessed chemical stability when the triple drug admixture was placed in a single container. In phase II, individual drug components of the admixtures were infused via the multiple line infusion system. Samples were collected at time 0, 1 hr, 2 hrs, 4 hrs, 12 hrs, and 24 hrs. All samples were frozen and stored at -70 degrees C until assayed. Samples were assayed using stability-indicating high performance liquid chromatography. The triple drug admixtures were considered to be chemically stable if there was < or = 10% loss of stated potency over 24 hrs. In phase I, chemical stability was observed for all triple drug admixtures at 24 hrs. In phase II, dobutamine, dopamine, norepinephrine, and sodium nitroprusside showed chemical stability at 24 hrs. Nitroglycerin showed a two-fold increase in concentration at 24 hrs compared with the initial concentration through the test infusion system; however, this amount was still one third lower than originally anticipated. All triple drug admixtures were chemically stable when placed in single containers. Dobutamine, norepinephrine, and sodium nitroprusside showed chemical stability when delivered via a multiple line infusion system. The reduced recovery of nitroglycerin from the test infusion system may result from adsorption of the nitroglycerin to the polyvinyl chloride plastic cassette and tubing during infusion.