In recent years increasing emphasis has been placed on the benefits of exercise to normal healthy adults as well as to those recovering from heart attacks and other illnesses. As an aid to exercise training many portable heart-rate monitors have been placed on the market but little appears to have been done to assess the performance of these machines. The authors have examined the performance of four such monitors both under bench-test and practical exercising conditions. When tested on the bench using electronic equipment, the machines rarely exhibited errors exceeding 2-3 bpm over a measurement range of 30-240 bpm. However, when tested with subjects walking or jogging at low speeds on a treadmill, typically 20-70% of the readings given by the machines had errors of greater than 20 bpm. In some cases over 50% of readings had errors exceeding 50 bpm.