1. Postural changes of plasma renin activity (PRA) before and after the administration of a beta-blocking agent (oxprenolol) were studied in nine patients with renal homotransplantation and in ten normal subjects.
2. In normal subjects PRA always increased during upright posture without any correlation with postural changes in mean arterial pressure. Oxprenolol reduced the postural increase of PRA.
3. A postural increase of PRA could be detected as early as 20–25 days after renal transplantation, and appeared with increasing frequency as time elapsed.
4. There was a significant inverse correlation (r−0.794,P <0.001) between the postural changes of PRA and those of mean arterial pressure.
5. These results suggest that in patients with renal homotransplantation the postural increase of PRA can only partly be due to circulating catecholamines or the sympathetic nervous system and may be explained by inverse changes in the mean arterial pressure.