In 9 trained athletes and 4 sedentary subjects the anaerobic threshold was assessed on a cycle ergometer, using the deflection point of heart rate in a protocol in which the workload increased by 10 W every 45 s. The workload at which plasma lactate concentration equalled 4 mmol.l-1 was assessed under steady state conditions on separate occasions. In addition, in 3 subjects the non-invasive anaerobic threshold and the 4 mmol.l-1 lactate level under steady state conditions were assessed on a treadmill. On the cycle ergometer 6 subjects demonstrated a deflection point in the heart rate record, whereas the others failed to do so. The workload at which heart rate departed from linearity in the progressive protocol did not coincide with the steady state 4 mmol.l-1 workload but occurred at a higher workload. On the treadmill no deflection in heart rate was observed. It is concluded that in cyclists a deflection in heart rate does not always occur, and when it does, it does not coincide with the anaerobic threshold determined under steady state conditions.