Representative deman cardiac pacemakers were tested with several common radiation therapy devices and ancillary equipment. The experimental set-up essentially duplicated the dosimetric and electroconductive environment of the radiation therapy patient bearing a pacemaker. Observations of pulse width and interval confirmed that pacemaker output was generally not appreciably affected by the simulated treatments. Occasionally, changes were caused by ancillary equipment and start-up of treatment. We conclude that although interference hazards are sometimes seen, radiation therapy is not contraindicated in patients wearing pacemakers.