Eleven patients with hypertension and renal artery stenosis of fibromuscular type were investigated with renal blood flow estimations with Xenon133 wash-out technique before and after alpha-receptor blockade with infusion of phenoxybenzamine in the stenosed renal artery. During the whole investigation frequent blood samples for renal vein renin estimations were drawn from both kidneys. Immediately after the Xenon injection renal vein renin elevations on the investigated side were observed in 9/11 patients. The renin peak was reached in 10 min and the mean increase for all patients was 152%. The mean duration of the peak was 19.25 min. A neurogenic renin release mechanism triggered by renal chemoreceptors and mediated by beta-adrenergic fibres was suggested. This finding with renin release stimulation at Xenon133 infusion in the renal artery is of great importance in judging results of investigations concerning renal blood flow and renin when the Xenon wash-out technique is used. Blood samples for renin estimations should not be taken within 25 min after intrarenal Xenon133 infusions.