1. The cardiovascular responses to lower body subatmospheric pressure or to standing were measured in the same nine normal male subjects after a 12 h and after a 48 h fast.
2. After the 48 h fast there was a significant reduction in diastolic blood pressure and forearm vascular resistance (relative to the values after a 12 h fast) when subjects were supine.
3. During exposure to lower body subatmospheric pressure, subjects who had fasted for 48 h showed an inability to maintain systolic blood pressure, accompanied by an impairment of forearm vasoconstriction and an exaggerated tachycardia (relative to their responses after a 12 h fast). Similar disorders of cardiovascular homoeostasis were seen on standing.
4. The results are consistent with an inhibition of sympathetic nervous activity after a 48 h fast, but other possibilities are discussed.