1. The influence of supine rest on the blood pressure response to standing and 70° head-up tilt was studied in detail for the first 30 s after the change of posture.
2. Following 20 min of supine rest, the active transition to standing was accompanied by an immediate increase in systolic pressure of 29 ± 6 mmHg (mean ± sem). This was followed by large fluctuations in systolic pressure: to −28 ± 2 mmHg below control after 7 s and to 22 ± 2 mmHg above control after 22 s (17 mmHg in excess of the systolic pressure level after head-up tilt).
3. Following 1 min of supine rest, there was no difference in the immediate increase in systolic pressure. However, the magnitude of the subsequent changes was significantly diminished.
4. With head-up tilt the immediate increase in blood pressure was absent and afterwards small changes were found that were also significantly influenced by the period of prior rest.
5. Taken in conjunction with earlier studies, the following mechanisms are suggested. The immediate blood pressure increase resulted from compression of arteries by the contracting postural muscles. The subsequent blood pressure fall was caused by at least two mechanisms: (a) the fall was predominantly of reflex origin, because the immediate pressure increase stimulated the systemic baroreceptors; (b) supine rest possibly augmented the translocation of blood from the thorax which contributed, approx. 5 s from standing, to the reflex fall of blood pressure.