Skin temperature was measured in the forearms of 8 healthy volunteers to study the effect of transdermal application of a stable prostacyclin analogue (Iloprost). Local skin temperature was significantly increased 24 hours and 48 hours after application of the drug when compared with an area of control skin. The effect had worn off by 72 hours. At a dose of 25 micrograms there were no systemic effects of vasodilatation or antiplatelet behaviour, but when the dose was increased to 75 micrograms there was a decreased rate of platelet aggregation for 24 hours. Histology of a skin biopsy suggested that the increase in skin temperature was due to vasodilatation and not acute inflammatory reaction. The results of this pilot study suggest that transdermal application of Iloprost is a suitable vehicle for administration of the drug and a prospective randomised trial is proposed for patients with Raynaud's Syndrome.