To induce luteal regression-related abortion/delivery and treat pyometra in dogs, various PGF(2alpha)- analogues (PGAs)mu are administered, but a PGA most appropriate for clinical application in dogs, with a low incidence of side effects, is being investigated. In this study, we compared the effects of etiproston tromethamine (PGA-E), which has not been investigated in dogs, with those of cloprostenol (PGA-C), which is routinely used in dogs. A single dose of PGA-E at 100, 200, 400 or 800 mug or PGA-C at 12.5, 25, 50 or 100 g was administered to beagles (n=5 per group) 25 days after ovulation, when the corpus luteum was in the functional phase. We compared the state of luteal regression by measuring plasma progesterone levels. As side effects, the incidences of salivation, vomiting, tachypnea, diarrhea and the drop in body temperature were investigated. In the 400-mug and 800-mug groups treated with PGA-E, the mean intervals from administration until luteal regression were 18.6 days and 31.2 days, respectively. In the dogs treated with 50mug or more of PGA-C, luteal regression was noted 2 days after administration. The above side effects were observed for 3 hr after administration of PGA-E/PGA-C. In the dogs treated with 800mug of PGA-E, the mean body temperature was 36.7degreesC 4 hr after administration; hypothermia persisted. PGA-E may be less useful than PGA-C for promoting luteal regression in dogs in clinical application.