The acute effects of hydroxyethylrutosides on capillary filtration were studied in 12 normal subjects, 25 patients with venous hypertension and 22 diabetics with microangiopathy. The two groups of patients randomly received a single oral dose (500 or 1000 mg) of hydroxyethylrutosides. A single dose of 500 mg was used for normal volunteers. In the following 6 hours capillary filtration was studied with straingauge plethysmography. The decrease in capillary filtration was evident within the first hour and was at its peak between the second and fourth hour. After 6 hours it was still significantly below baseline values in patients. The 1000 mg dose was significantly more effective in both groups of patients. This study confirms the efficacy of hydroxyethylrutosides in decreasing capillary filtration. It suggests that the effect of one dose lasts at least 6 hours and also that the higher dose is more effective.