SUMMARY Changes of serum renin activity, heart rate, blood pressure, renal sodium, potassium and metadrenaline excretion and alveolar gas tensions were recorded during two 2–3 h exposures to 446 mmHg barometric pressure (simulated altitude of 4279 m or 14000 ft). Serum renin activity rose considerably during the first exposure and only slightly during the second. This effect was positively correlated with the changes of heart rate. There was little change in blood pressure or in the rate of renal excretion of sodium, potassium or the metadrenalines. It is suggested that the change of serum renin activity cannot be explained by a direct effect of hypoxaemia, emotion, posture or the diurnal rhythm of renin secretion but that it may be correlated with changes of cardiovascular function.