To the Editor: It is obvious that in the study of Santiago et al.,1 2 mg of naloxone corrected the inability to compensate for an added respiratory load in patients with chronic obstructive pulmonary disease (COPD). However, this study does not provide sufficient evidence of the involvement of the endorphin system in mediating this compensatory response. Naloxone is a competitive inhibitor of opiates. Its inhibitory concentration depends on the level of agonist in the receptor vicinity.2 Naloxone has agonist activity3 4 5 as well as an ability to modify the effects of a variety of non-opiate drugs when it is given in. . .