It has been shown (Robinson, 1951) that in the ewe the number of ova shed can be increased by the injection of a single dose of mare serum gonadotrophin (P.M.S.) if this is given during the follicular phase of the oestrous cycle. The investigations described here were made to examine the usefulness of this treatment for increasing the lamb crop in commercial flocks.

Observations were made on 594 ewes of 3 breeds in 11 flocks (Romneys, 9; Cheviot, 1; Southdown, 1). Most ewes were purebred from old-established flocks. The Romneys were running on the marshlands and mixed farms of Kent and East Sussex, the South-downs were near Glynde, in Sussex, and the Cheviots were on a farm near Brampton in Cumberland. Management within the flocks followed the practice normal for the different breeds. Most ewes were 2½ to 4½ years old at the start of treatment; a few younger ewes (1½ years) and some aged ewes were also involved.