A possible contribution of circulating kinin for orthostatic hypotension (OH) in diabetic patients was investigated using a sensitive RIA of plasma bradykinin (BK). Thirty six diabetics underwent 10 min of orthostasis, in which 20 patients developed OH. The baseline plasma BK was 15.8 +/- 1.8 pg/ml in diabetics with OH, which increased to 31.0 +/- 5.6 pg/ml (p less than 0.05). The basal plasma BK was 19.4 +/- 4.1 and 17.8 +/- 2.3 pg/ml in normal subjects (n = 10) and in diabetics without OH (n = 16), respectively. These values were not significantly different from the diabetics with OH. In response to standing there were no significant changes in plasma BK in both groups. There was a significant correlation between the absolute increment of plasma BK and the decrement of systolic blood pressure (BP) when all diabetic patients were compared (r = -0.364, p less than 0.05). The baseline PRA and plasma norepinephrine (NE) were similar in normal subjects and diabetics with or without OH. In response to standing there were approximately two-fold increases in PRA and plasma NE. There was no significant correlation between plasma NE increment and the fall in systolic BP. These results suggest that kinins may participate in OH in diabetic patients.