Many endocrine abnormalities were reported in patients with chronic renal failure (CRF), including elevated ANP, but little is known on ANP vaso-regulatory effects during exercise. This study is aimed to find whether plasma ANP rises during exercise in patients with CRF, as it does in normal subjects, and whether there is a difference between this response in both those groups. 36 subjects were studied, divided into three groups: I--CRF patients treated conservatively (nondialyzed), II--CRF patients, dialyzed, III--healthy subjects. Plasma ANP was measured at rest and then immediately after submaximal treadmill work load according to modified Bruce protocol with continuous ecg and blood pressure monitoring. In the CRF nondialyzed patients the metabolic equivalent (MET) was significantly lower than in the other groups. The myocardial oxygen consumption, estimated as double product quotient, was the lowest in the nondialyzed CRF patients. A significant increase of the plasma ANP was seen in all subjects studied, significantly higher in the CRF patients than in the control healthy subjects. No correlations were found between the plasma ANP level and other measured parameters.