In an attempt to resolve conflicting reports about the identity of a low-renin subgroup in essential hypertension, the distribution of plasma-renin activity (P.R.A.) has been examined in 82 hypertensive subjects before and after stimulation with bendrofluazide. The unstimulated basal P.R.A. showed no evidence of a separate subgroup of patients with low P.R.A. values although the distribution was slightly skewed with a tail to the right when compared with 83 normotensive subjects. In 38 of the patients post-stimulation P.R.A. observed in age and sex matched normotensive controls exposed to the same stimulus. However the percentage rise in P.R.A. in all 83 patients was less than half that of the controls. Thus the identification of a low-renin subgroup of hypertensive patients is critically determined by the standard of comparison employed: if the percentage rise is considered subnormal responsiveness is a feature of essential hypertension. If absolute post-stimulation values are used, there is a substantial group of patients with "low renin hypertension". The demarcation of such a group is, however, essentially arbitrary.