Nine male endurance runners were evaluated with bicycle exercise testing before a training break of 3 weeks duration, and 0, 2 and 4 weeks after resumption of training to assess the effects of training on resting and exercise plasma atrial natriuretic factor (ANF) measured at 50% and 100% of predetermined maximal workload. Maximal oxygen uptake and lean body mass (LBM) were calculated at each time point. Maximal oxygen uptake decreased during training break, but rose 4 weeks after resumption of training (P less than 0.01). LBM was unchanged after inactivity, but rose after resumption of training (P less than 0.01). Plasma ANF at rest did not change throughout the experiment. ANF levels rose after training break at maximal workload (P less than 0.05), and decreased 4 weeks after resumption of training, but only at submaximal workload (P less than 0.05). No correlations between systolic blood pressure, mean blood pressure or heart rate and ANF could be demonstrated. These results indicate that the haemodynamic changes associated with endurance training are reflected in plasma ANF levels during exercise, but not at rest. The full adaptation of ANF release to training probably requires more time than the 4 weeks reported for the haemodynamic adjustments.