1. Forearm blood flow and cardiac output were measured during 12–14% oxygen breathing in eleven subjects aged 23–72 years. Forearm blood flow alone was measured in eight additional subjects aged 45–70 years.
2. The cardiac output increased and systemic vascular resistance decreased during hypoxaemia in all subjects. Although the initial cardiac output was lower with increasing age, the change during hypoxaemia correlated poorly with age.
3. Forearm blood flow increased with age and the change produced by hypoxaemia was also related to the age of the subjects. In six healthy subjects aged 23–37 years forearm blood flow increased during hypoxaemia. In thirteen older subjects, six of whom had chronic obstructive pulmonary disease, forearm blood flow either increased to a lesser degree, decreased, or remained unchanged during hypoxaemia. The mechanism for the change in forearm vascular response to induced hypoxaemia in older subjects was not apparent from these studies.