Ultrastructural morphology was monitored at intervals of time in corpora lutea of superovulated ewes following treatment with Prostaglandin F2-alpha. Lutein cells displayed an initial increased metabolic rate as marked by lipid accumulation, spherical mitochondria and a high respiratory rate. Three hours following injection, complete disorganisation of the cell was apparent; mitochondria are ruptured and a clear state of autolysis exists. The possible mode of luteolytic action of Prostaglandin F2-alpha in the ewe is discussed.