Production of prostaglandin F in ewes following luteal regression induced with a prostaglandin analogue, Estrumate (Cloprostenol; I.C.I. 80996): Challis, J.R.G., C.S. Forster, B.J.A. Furr, J.S. Robinson and G.D. Thorburn: Prostaglandins, 11:3, 537, 1976. University of Oxford, Nuffield Institute for Medical Research, and John Radcliffe Hospital, Dept. of Obstetrics and Gynaecology, Headley Way, Headington, Oxford