This invention relates to a stable epoprostenol composition that can be combined with commercially available IV fluids and can be administered in its reconstituted and/or diluted form under ambient conditions of about 15-30.degree. C. for greater than 24 hours. The composition preferably contains (a) epoprostenol or a salt thereof; (b) a alkalinization agent; and (c) a base, such that when reconstituted or in solution, the solution has a pH>11. Methods for making the lyophilized composition are also disclosed.