1. [U-14C]glucose was infused intravenously into conscious lactating goats exposed to thermoneutral or cold environments for a total of 24 h. The irreversible loss of glucose from the whole body and uptake of glucose by the mammary gland was measured and glucose utilization in the udder was studied by measuring the incorporation of radioactivity into carbon dioxide in mammary venous blood, into milk lactose and milk triglyceride-glycerol. 2. Exposure to cold increased the circulating level of glucose and slightly though not significantly, increased the non-mammary irreversible loss of glucose. 3. The extraction of glucose from the circulation by the udder fell, the secretion of lactose by the udder also fell and this correlated closely with the reduced secretion of milk in the cold. 4. Simultaneous measurements of the concentrations of insulin, growth hormone and corticosteroids in the arterial plasma were made. 5. The plasma concentration of corticosteroids increased significantly in the cold. 6. It is concluded that reduced glucose uptake and lactose synthesis by the udder are important factors which reduce milk secretion during cold exposure.