The following experiments were made to investigate whether any changes in water balance and in the release of arginine vasopressin (AVP) accompany the development of thermal adaptation. Twelve guinea-pigs (300-400 g initial weight) were kept in individual metabolic cages at 22 degrees C during weeks 1 and 5. During weeks 2-4, six of them were exposed to 5 degrees C, and six to 28 degrees C. Before the start of the experiment, eight animals were implanted with chronic arterial catheters for removal of blood samples. Food and water intake, body weight, and colon temperature, as well as the amounts of urine and feces, were recorded in each animal every morning. In urine and blood plasma samples (taken daily, resp. weekly), the osmolality was estimated by vapor pressure osmometry, and concentrations of AVP by a radioimmunoassay. It is apparent that the daily turnover of water increased from 94 ml in guinea-pigs adapted to 22 degrees C (N), to 111 ml in cold adapted (CA), and to 154 ml in warm adapted (WA) animals. In CA the amounts of AVP excreted in urine increased dramatically (being 10 times higher than in WA). This high release of AVP cannot be explained by changes in osmotic pressure and by alterations in volume of extracellular fluid. It is concluded that AVP is released in CA guinea-pigs mainly as a stressor hormone, in amounts which highly exceed the antidiuretic needs. The WA animals, having free access to water, did not use the AVP system to conserve water. They doubled their water intake, producing more urine of lower osmolality, corresponding to the reduced release of AVP.