1. Following single intramuscular doses of [14C]fluprostenol (0.5--2.4 micrograms/kg) to three female horses and to three gelded male horses, radioactivity was present in the plasma within 5 min; peak concn. (0.32--1.30 ng/ml fluprostenol equiv.) occurred 5 to 90 min after injection. Radioactivity was still present in the plasma of the females after three days. About 88% of fluprostenol is bound to plasma proteins. 2. Radioactivity was present in the parotid saliva of the gelded male horses within 10 min. Peak concn. (45--91 pg/ml fluprostenol equiv.) occurred from 5 min to 1 h after injection. Saliva : plasma concn. ratios varied inversely with saliva flow rate and limiting ratios were 0.33 and 0.41 for the combined results of two experiments on each of two male horses; the calculated value is 0.46 Chromatography indicated that the majority of plasma and saliva radioactivity was [14C]fluprostenol. 3. Excretion of radioactivity in the urine was rapid and virtually complete 12 h after dosing. The total radioactivity excreted in urine by the female horses was 45% of the dose (96 h) and by the gelded male horses 53% (30 h). About 30% of the radioactivity present in the urines was unchanged fluprostenol. 4. Faecal excretion, which was substantially complete after 2 days, accounted for 32% of the radioactivity administered to the female horses. 5. Tissue conc. of radioactivity in the female horses at four days were below the limits of detection (90 pg/g), but 0.2--0.9% of the dose was detected at the site of injection.