Dogs were immersed in cold water 1 h/day for 30 consecutive days. During the first immersion, oxygen uptake increased approximately sevenfold and colonic temperature decreased; a large increase in plasma epinephrine and norepinephrine concentrations was found; plasma glucose, lactic acid, and free fatty acid (FFA) concentrations were also increased. An adaptative response to cold was evidenced during the 10th immersion by a further increase in oxygen uptake and a reduction in the fall of colonic temperature; plasma epinephrine, glucose, and lactic acid were diminished whereas FFA were higher. Consequently 10 h spent in cold are sufficient to induce an improved capacity for heat production. An enhanced ability to produce nonshivering thermogenesis mediated by epinephrine or norepinephrine is not obvious because after the 30th immersion no enhanced calorigenic response to epinephrine or norepinephrine infusion was found. In dogs mechanisms other than norepinephrine-enhanced sensitivity might be involved in this kind of cold adaptation.