The dominant follicle of oestrous ewes was injected with saline or indomethacin. Treated follicles were obtained at 2 and 10 days after treatment and examined histologically. Indomethacin-injected follicles were abnormally enlarged and remained unruptured. On Day 2 after treatment, these follicles appeared grossly hyperaemic. The follicular walls of control and indomethacin-treated groups had luteinized. Blood samples for the measurement of serum progesterone were collected from animals with their ovaries left intact. Systemic progesterone and oestrous cycle lengths were unaltered by drug administration. It appears that prostaglandins are essential for ovulation in the ewe. The occurrence of ovulation per se was not a prerequisite of ensuing luteal-phase normality.