A sustained-release administration form of oral ritodrine was introduced to cope with the problem of the short dosage interval inherent to conventional oral ritodrine administration. The bioavailability of this application form was assessed in a clinical study including 11 patients. With a dosage frequency of only 3 times daily and a dosage form index of 1.8, plasma levels of 14.6–26.5 ng/ml ritodrine (equivalent to an infusion rate of approximately 50 µg/min) were obtained without serious cardiovascular or metabolic side effects. It was concluded that with this new formulation clinical applicability of orally administered ritodrine in tocolytic therapy has increas