The degree and duration of miosis, induced by low doses of parenteral morphine (65 mcg/Kg i.m.) before end after ten days of naloxone treatment, were evaluated in eleven volunteers affected by migraine without aura. Morphine induced miosis was significantly more Intense and persistent after chronic naloxone treatment than before the drug was administered. In another group of seven volunteers suffering from migraine without aura, no differences between morphine-induced miosis, prior to and after a ten-day treatment with placebo, were observed. In addition, a third group of fifty-four subjects, suffering from migraine without aura, underwent three-month naloxone treatment (150 mcg/Kg/i.m./day). All subjects Included in the third group, were partially or totally refractory to conventional therapy. Pupillopharmacological results indicate that the benefit gained from chronic administration of the opiate antagonist may be related to some type of naloxone-induced supersensitivity of the opioid receptor population.