Seven dairy cattle showing clinical post partum endometritis were monitored daily by rectal examination of uterus, ovaries and vagina and by sampling of jugular venous blood and uterine pus. The animals were treated by intramuscular injection of 500 microgram cloprostenol (ICI). All seven animals had an immediate reduction of plasma progesterone concentrations and showed oestrus within two to three days. In six cases the clinical endometritis resolved, with uterine involution, within seven days and uterine mucus sample showed no specific growth. One animal required a second cloprostenol injection 11 days after the first to achieve complete resolution. Subsequently, plasma progesterone levels rose in all animals but two cases later appeared to become anoestrous, perhaps due to pituitary insufficiency. In a further study of chronic endometritis in cattle in the field, 51 out of 56 animals treated in the presence of a corpus luteum showed oestrus within 14 days with resolution of the condition; five others did not respond. None of the three cases which had been treated in the absence of a corpus luteum showed any improvement. This lack of response was not unexpected and supports the view that the mode of action of cloprostenol in this treatment regime is to induce luteolysis. The mechanism of therapy and possible explanation for the occurrence of chronic endometritis are discussed.