1. Six elderly (66-71 years) and six young (20-23 years) subjects (half of each group women) were cooled for 2 h in moving air at 18 degrees C to investigate possible causes of increased mortality from arterial thrombosis among elderly people in cold weather. Compared with thermoneutral control experiments, skin temperature (trunk) fell from 35.5 to 29.5 degrees C, with little change in core temperature. 2. Erythrocyte count rose in the cold from 4.29 to 4.69 x 10(12)/l, without a change in mean corpuscular volume, indicating a 14% or 438 ml decline in plasma volume; increased excretion of water, Na+ and K+ accounted for loss of only 179 ml of extracellular water. 3. Plasma cholesterol and fibrinogen concentrations rose in the elderly subjects from 4.90 mmol/l and 2.97 g/l (control) to 5.45 mmol/l and 3.39 g/l in the cold, and in the young subjects from 3.33 mmol/l and 1.84 g/l (control) to 3.77 mmol/l and 2.07 g/l in the cold. Increases were significant for the elderly subjects, the young subjects and the group as a whole, except for cholesterol in the young subjects, and all were close to those expected from the fall in plasma volume. 4. Plasma levels of Protein C and factor X did not increase significantly in the cold in the elderly subjects, young subjects, or the group as a whole. 5. The results suggest that loss of plasma fluid in the cold concentrates major risk factors for arterial thrombosis, while small molecules, including protective Protein C, redistribute to interstitial fluid.