1. The cardio-renal haemodynamic and endocrine effects of the anti-hypertensive agents 2-(2,6-dichlorophenylamine)-2-imidazoline hydrochloride (clonidine, St 155) and 2-(5-fluor-o-toluidine)-2-imidazoline hydrochloride (St 600) were investigated in twelve patients with essential hypertension.

2. The anti-hypertensive action of both compounds was similar and was accompanied by a reduction in heart rate and in cardiac output, total peripheral resistance being unchanged. There was no alteration in renal blood flow and glomerular filtration rate, and renal vascular resistance showed a significant decrease. Blood volume and plasma renin concentration did not change significantly. An inverse relation between changes in plasma volume and plasma aldosterone concentration was observed. In the face of similar reductions in blood pressure, no differences were observed between systemic and renal haemodynamic and endocrine responses after clonidine and St 600.

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