Cows were treated with various doses of pregnant mare serum gonadotrophin given either towards the end of the normal cycle, or prior to expression of the corpus luteum. The animals were kept under normal farm conditions; the object of treatment was to obtain twin ovulations and calvings.

Some success was achieved, but of the treatments tried none could be considered commercially worth while; many variants of treatment are possible and much more requires to be done. For practical application it is important that a treatment should not, or only rarely, result in more than three ovulations. The few cows reported in this paper which were pregnant and which had more than three corpora lutea aborted at about 5 months.

This work was carried out from the School of Agriculture, Cambridge, during tenure of a grant from the Agricultural Research Council. It was possible only through the help of those farmers who provided animals for experiment; I here record my indebtedness to them.