The infusion of Salmonella typhimurium endotoxin into pregnant mares resulted in a biphasic release pattern of PGF-2 alpha as determined by 15-keto-13,14-dihydro-PGF-2 alpha concentrations. The initial phase of 1 h duration was followed by accentuated release by 2 h after infusion; concentrations reached basal levels by 6 h. In 7 mares at 23, 26, 29, 33, 36, 53 and 55 days of gestation, fetal death occurred between 36 and 120 h after infusion; 12 mares at 46, 51, 56, 59, 65, 71, 73, 85, 103, 138, 283 and 318 days of gestation did not abort after endotoxin infusion. Luteal activity was compromised in all mares by 9 h after infusion. Progesterone concentrations were consistently lower in mares that aborted (1-2 ng/ml) than in those that did not abort. Mares therefore appear to be vulnerable to fetal loss by a clinical syndrome induced by Salmonella typhimurium endotoxin until about 50-60 days of gestation.