We investigated the effect of glucose and glucose polymer ingestion on plasma arginine vasopressin (pAVP) levels, on plasma osmolality (p-osm), and on performance during two prolonged endurance events. The study subjects were 37 Finnish elite endurance athletes, of whom 18 were orienteers and 19 cross-country skiers. Plasma AVP increased in both combined glucose and glucose polymer groups, but the increase in the glucose polymer group was significantly smaller (P less than 0.001) than that in the glucose group. A significant change in p-osm caused a significant change in pAVP and vice versa. Both the orienteers and the skiers on glucose polymer tended to have more success in the competition; the orienteers on glucose polymer ran the last third of the competition significantly faster than those on glucose (P less than 0.05). It is suggested, in the light of the smaller pAVP response, that after glucose polymer ingestion the physical stress in prolonged endurance exercise is smaller than after ingestion of glucose.