The response of the bovine ovary to single subcutaneous injections of pregnant mares' serum (P.M.S.) and horse pituitary extract has been studied both quantitatively and qualitatively. For the former both time-response and dose-response data were obtained, for the latter time-response data only. Quantitative changes were measured by the following ovarian indices: mean follicular diameter (M.D.); percentage of follicles M.D. $\geq $ 10 mm.; number of follicles M.D. $\geq $ 4 mm.; number of follicles M.D. $\geq $ 10 mm.; total follicular volume, and weight. Only the first two criteria were found to have more than a rough relationship with the duration and magnitude of the treatment given. The threshold dose of P.M.S. for quantitative effects was between 1000 and 2000 i.u. For higher doses the ovarian response was most pronounced, but there was evidence that the maximal rate of stimulation had been reached with doses of 3000-4000 i.u. For both gonadotrophins the stage of the oestrous cycle at which injections were given had no differentiating effect upon the quantitative response obtained. In particular, the total number of follicles stimulated to growth, an index that ranged widely, showed no correlation with this variable. Qualitative changes observed included multiple ovulation, anovulatory luteinization, the formation of abnormally small corpora lutea (P.M.S. only), and the occurrence of haemorrhagic follicles. An effect caused by horse pituitary injections only was the rupture of a single follicle within 1-2 days of treatment ('shock' effect). Ovulation following treatment with P.M.S. only occurred when the injection was given in the last, or follicular, phase of the oestrous cycle. With horse pituitary injections ovulation followed treatment given at all stages of the cycle. The differences in the biological properties of these two gonadotrophic preparations have been discussed in the light of the new evidence arising from this study.