Objective The aims of this study were to examine (i) base line pressure pain thresholds in patients with chronic fatigue syndrome and those with chronic low back pain compared with healthy subjects, (ii) the change in mean pain threshold in response to exercise, and (iii) associations with exercise induced increase in nitric oxide Participants Twenty six patients with chronic fatigue syndrome suffering of chronic pain, 21 patients with chronic low back pain and 31 healthy subjects Methods Participants underwent a submaximal aerobic exercise protocol on a bicycle ergometer, preceded and followed by venous blood sampling (nitric oxide) and algometry (hand arm calf low back) Results Patients with chronic fatigue syndrome presented overall lower pain thresholds compared with healthy sub jects and patients with chronic low back pain (p