The efficacy of topical prostacyclin as an antithrombotic agent was tested in a model of microvascular trauma. Preliminary measurements were made to determine a suitable dosage. Twenty-seven central arteries of rabbit ears were then prepared and 32P-labelled platelets infused intraaortically. Arteriotomy (7 mm) was followed by intimectomy (5 mm). Fifteen vessels in a control group were irrigated with Ringer's lactate and 12 vessels in an experimental group were treated with prostacyclin (10 ng/ml) in normal saline. Bleeding times at the sites of arteriotomy intimectomy, in vivo accumulations of isotope-labelled platelets, amounts of red thrombotic material, and patency were recorded. Patency was lower following prostacyclin treatment (1/12 as against 5/15) but not significantly so, and there were no statistically significant differences in other parameters. Prostacyclin treatment decreased vessel wall tone, interfering with blood-flow and promoting thrombus formation.