1. Mean deep body temperature fell by 0.4 ± 0.1 (sd) °C in five sedentary, clothed 63–70 year old men and by 0.1 ±0.1°C in four young adults after 2 h exposure in still air at 6°C (P < 0.001).
2. The mean increase in systolic and diastolic pressure was significantly greater (P < 0.002) in the older subjects (24 ± 4 mmHg systolic, 13 ± 4 mmHg diastolic) than in the young (14 ± 6 mmHg systolic, 7 ± 3 mmHg diastolic) after 2 h at 6°C. A small rise in blood pressure occurred in the older men at 12°C, but there was no increase in either group at 15°C.
3. The association of variables is particularly marked between systolic blood pressure and core temperature changes at 6°C.
4. There were no appreciable cold-adaptive changes in blood pressure or thermoregulatory responses after 7–10 days repeated exposure to 6°C for 4 h each day.
5. Blood pressure elevation in the cold was slower but more marked in the older men. These changes in blood pressure may provide a possible basis for delineating low domestic limiting temperature conditions.