Blood pressure of rabbits was decreased 20-30% by intravenous administration of prostaglandin E1. The blood flows of the ciliary body and choroid were measured using the hydrogen gas clearance method. The ciliary body blood flow was 92.6 +/- 9.3 ml/min/100 g before administration, and 91.6 +/- 18.8 after. The choroidal blood flow was 75.6 +/- 10.0 and 73.9 +/- 6.6, respectively. There were no significant differences in blood flow before and after administration of prostaglandin E1. Partial pressure of arterial blood carbon dioxide and oxygen, pupil size and ocular tension were also measured against time. Ocular tension fell slightly from 45 to 60 minutes after administration, but the other levels showed no significant differences. Since ocular blood flow displayed no significant changes in spite of the decreased systemic blood pressure, prostaglandin E1 may be useful as a hypotensive drug in ophthalmic surgery.