SummaryAn epidemiological survey amongst 3,001 subjects between the ages of 45 and 64 in the Burgh of Renfrew, Scotland, revealed that 468 (15.6 %) subjects had a diastolic blood pressure of 100 mmHg or more. Examination of the General Practitioner's medical records of those subjects whose diastolic blood pressure was less than 100 mmHg revealed 55 subjects who were receiving antihypertensive treatment and a further 113 subjects who were receiving no antihypertensive therapy, but who had previously been recorded as having a blood pressure above this level (Total 636 ; 21.2 % of survey).Investigation of the severest cases of hypertension has demonstrated no cases of primary hyperaldosteronism and a frequency distribution curve for plasma angiotensin II concentration among this group shows no evidence of a subpopulation of subjects with low levels.