Prostaglandin F2α (PGF2α) has been shown to possess luteolytic properties in several species including the rabbit (Duncan & Pharriss, 1970; O'Grady, Kohorn, Glass, Caldwell, Brock & Speroff, 1972). Surgical removal of the corpus luteum leads to parturition in the rabbit. Thus, PGF2α should initiate parturition in this species. In primates, controversy has arisen over the ability of PGF2α to produce luteolysis (Kirton, Pharriss & Forbes, 1970). Since prostaglandins are very rapidly cleared by the pulmonary circulation, observed discrepancies may be due to the actual route of administration and dosage used. Experiments have been carried out to measure the effect of PGF2α in producing parturition in the pregnant rabbit. PGF2α was infused continuously into rabbits with an indwelling aortic catheter (Nathanielsz & Abel, 1972). Infusions were started on day 21 and continued until delivery. PGF2α was infused at dose rates varying from 1·125 ng/h