Changes in plasma 15-keto-13, 14-dihydro-prostaglandin F were monitored at frequent intervals before, during and after spontaneous deliveries (three mares) and foalings induced by oxytocin (eight mares). No evidence of increased concentrations of the prostaglandin metabolite was observed in the final 10 days of gestation. In spontaneously delivering mares, there was a marked increase from 3 ng/ml at −125 mins to 18 ng/ml at −65 mins to the highest observed value of 182 ng/ml at 20 mins pre-partum. Following delivery, concentrations declined rapidly to around 0.2 ng/ml. Further release of prostaglandins was seen on Days 1 and 3 post partum. In oxytocin induced mares, maximal concentrations of about 100 ng/ml were observed to occur very close to the time of delivery. Large increases were observed as early as 2 mins following oxytocin injection. The significance of these findings is discussed in relation to parturient and post parturient events and changes in levels of the hormone relaxin during the same period.