We investigated the effects of i.v. magnesium on secondary hyperalgesia following heat/capsaicin stimulation in human volunteers. Twenty-five volunteers were included in this double blind, randomized, crossover study. Sensitization was induced in the volunteers, who were then subjected to either i.v. saline or magnesium sulphate. No analgesic or antihyperalgesic effect could be demonstrated in sensitized skin during infusion of magnesium. In contrast, painfulness of thermal stimulation was increased in normal skin. These results suggest that i.v. magnesium has no important analgesic effects in clinically relevant doses.