The synthesis of radioactive nileprost, tritium-labelled at positions 18 and 19, and its application to the pharmacokinetics and biotransformation of this chemically stable prostacyclin derivative in the rat is described. 3H-Nileprost was absorbed after oral administration with a half-life of 23 min reaching maximum concentrations in the plasma 90 min after treatment. After intravenous injection there was a threephasic decline in plasma levels with half-lives of 15 min, 0.9 h and 11 h, respectively. Unchanged drug was eliminated with t 1/2 = 14 min. Brain levels of drug or metabolites were less than 5% of corresponding plasma concentrations. In autoradiographs after i.v. and oral administration a very low volume of distribution was found with maximum levels in the liver, the kidney and the stomach mucosa. Nileprost was very rapidly excreted, mainly by biliary elimination. Ten metabolites were detected in urine and bile, one of them being formed exclusively in the gut wall. The main fraction of 3H-activity in urine and bile, however, was due to unchanged drug.