1. In the summer of 1958, 15 mg. of hexoestrol was administered to Blackface wether lambs in one of the following ways; 5 mg. before weaning followed by two 5 mg. tablets at weaning, three 5 mg. tablets at weaning, and thirdly a single 15 mg. tablet at weaning. In a further series of experiments in 1959 the effect of both raising the pre-weaning implantation to 10 mg., and the total hexoestrol dosage to 20 mg. was studied.
2. Suckled lambs implanted with 5 mg. of hexoestrol at the milk clip gained significantly more in live-weight (18%) up to weaning in 1958 than did untreated lambs; in 1959 the difference was only 11%.
3. Between weaning and the end of the trial all treatments with the exception of that in which hexoestrol dosage was raised to 20 mg. showed a very highly significant increase in live-weight gain over untreated lambs, the increases ranging from 26 % to 54%.
4. In neither year was the overall gain in live-weight from pre-weaning to the end of the trial significantly increased by implanting part of the hexoestrol dose prior to weaning.
5. Live-weight gain of the lambs was not significantly increased when three 5 mg. tablets of hexoestrol replaced a single 15 mg. tablet.
6. The implanting of lambs with 20 mg. hexoestrol failed to give a response superior to that obtained with 15 mg. hexoestrol.
7. Each group of lambs receiving hexoestrol treatment showed an increased average carcass weight.