1. Neurogenic activity at rest was estimated from the immediate depressor response to intravenously administered trimetaphan in two groups of untreated hypertensive patients (thirty with essential hypertension and twenty with renal arterial stenosis). Responses were correlated with simultaneously determined haemodynamic functions and intravascular volume.

2. The two groups did not differ significantly in degree of response but the pattern of its correlations was different. In essential hypertension, the depressor response correlated directly with control total peripheral resistance (P < 0.005) and diastolic arterial pressure (P < 0.025) and inversely with blood volume (P < 0.02); data from seven age-matched normotensive subjects fell along the same regression lines. In renovascular hypertension, on the other hand, the response did not correlate with either pressure, resistance or volume.

The results suggest the interference in renovascular hypertension of a factor or factors not evident in essential hypertension. Further, this approach indicates that different types of hypertension may differ more by the pattern in which physiological factors interrelate than by a disturbance of a single factor alone.

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