Department of Veterinary Clinical Studies, University of Liverpool, Leahurst, Neston, Cheshire, and Imperial Chemical Industries Limited (Pharmaceuticals Division), Mereside, Alderley Park, Macclesfield, Cheshire (Received 14th June 1974) Tervit, Rowson & Brand (1973) have described the induction of fertile oestrus in cattle by intramuscular injection of a low dose (1 mg) of I.C.I. 79,939, an analogue of prostaglandin F2α, (Binder & co-authors, 1974; Dukes, Russell & Walpole, 1974) but neither this compound nor prostaglandin (PG) F2α, given in a single dose at any one time, will induce oestrus in all cattle in a herd since animals treated before Day 5 of the oestrous cycle will not respond (Lauderdale, 1972; Liehr, Marion & Olsen, 1972; Rowson, Tervit & Brand, 1972a, b; Hill, Dickey & Henricks, 1973; Cooper & Furr, 1974). Cooper (1974) and Cooper & Furr (1974) have described a regimen which overcomes this difficulty and provides a potentially valuable