1. The effect of locally applied subatmospheric pressure on subcutaneous blood flow was studied in six patients with acute myocardial infarction.
2. Blood flow was measured by the local 133Xe washout technique.
3. Application of subatmospheric pressures of −80 and −150 mmHg to the labelled area induced no vasoconstriction on day 1. On day 7 pressures of −40, −80 and −150 mmHg induced a decrease in blood flow of about 40–50%.
4. The absence of a vasoconstrictor response to locally applied subatmospheric pressure on day 1 could not be due to decreased venous distensibility. The underlying mechanism may be neuronal inhibition, probably antidromic, in sympathetic fibres.