Spheronized cores produced by extrusion and marumerization were microencapsulated with ethylcellulose by organic phase separation to produce beads exhibiting controlled-release characteristics. In vitro dissolution studies indicated that the drug was released as a first-order model and that the release rates were proportional to the amount of film on the bead. The bronchodilator activity in the anesthetized dog and the heart rate effect in the unanesthetized trained dog were evaluated. Microencapsulated beads were prepared which produced controlled release as assayed by bronchodilation. The heart rate increases induced by the controlled-release formulations were gradual in onset, and the total increase in heart rate over a 6-hr period was less than that associated with the plain drug powder.