1 The essential features and mode of action of oral osmotic drug delivery systems (Oros®) for metoprolol fumarate and oxprenolol succinate are described. 2 Critical aspects in the development of systems for once-daily administration of both drugs are discussed, and methods for evaluating in vitro release characteristics are presented. 3 In vitro testing confirmed that drug delivery corresponded closely to the theoretical release behaviour predicted from the physicochemical and membrane permeability characteristics for both oxprenolol and metoprolol systems. 4 In vitro release rates were also shown to be unaffected by pH, in vitro test procedures, dissolution media and long-term storage at different temperatures.