Twenty-four unfit volunteers and twenty-three superfit athletes were subjected to a progressive intensity treadmill exercise to total exhaustion in order to study the plasma met-enkephalin response to exercise. Blood samples were collected before and 5 min post-exercise. The basal met-enkephalin levels were significantly higher in the superfit individuals (180 fmol.ml-1) than in the unfit individuals (126 fmol.ml-1). Post-exercise the increase from basal levels of plasma met-enkephalin was significantly higher in the superfit athletes (180-278 fmol.ml-1) than in the unfit individuals (126-157 fmol.ml-1). The magnitude of the met-enkephalin responses to exercise therefore appears to be dependent on the amount of work performed and the degree of previous physical training. Peripherally circulating opioid peptides may, therefore, possibly play a role in the body's adaptation to exercise training.