Glomerular filtration rate and renal plasma flow (constant infusion technique using 125I-iothalamate and 131I-hippuran) were measured twice within a 1-week interval in nine young males with insulin-dependent diabetes of short duration (2-5 years). The study was performed in a randomized double-blind design, with the patients receiving either indomethacin (150 mg/day) or placebo for 3 days before the study. Measures of metabolic control did not change. No differences were found in glomerular filtration rate (144 +/- 9 versus 144 +/- 9 ml/min X 1.73 m2, mean +/- S.E.M.) or renal plasma flow (579 +/- 43 versus 560 +/- 52 ml/min X 1.73 m2), when measured during placebo or indomethacin treatment, respectively. It is concluded that the steady-state enhancement of glomerular filtration rate and renal plasma flow found in early insulin-dependent diabetes is not due to an excessive activity of the prostaglandin system.