In an attempt to reveal the importance of prostaglandins in the control of regional blood flow 20 mg/kg b.wt. indomethacin was given i.v. in conscious resting rabbits. Regional blood flow determinations were made before and 20 min after the injection using the labelled microsphere technique. The blood flow in the stomach wall was reduced by 0.75 +/- 0.17 g . min-1 . g-1 from a level of 1.64 +/- 0.24 g . min-1g-1. In jejunum the corresponding figures were 0.44 +/- 0.12 and 1.26 +/- 0.17 and in the brain 0.29 +/- 0.10 and 1.24 +/- 0.10. The blood flow in the liver via the hepatic artery increased by 0.20 +/- 0.02 g . min-1 . g-1 from a level of 0.13 +/- 0.02 g . min-1 . g-1. In the retina there was a reduction in blood flow by 2.75 +/- 1.03 mg . min-1 from a starting level of 15.1 +/- 2.3 mg . min-1. In a number of other tissues investigated there were no significant effects of the drug. The results suggest that under resting conditions prostaglandins play a role in the control of blood flow in the gastrointestinal tract, the brain and the retina--tissues which are likely to be rather active under such conditions.