1. The results of four experiments on the effects of oral and implanted hydroxyzine on lambs are reported.
2. The effect of hydroxyzine on daily rate of live-weight gain ranged from +7·8% to −6·2% improvement with a mean value of +2·3%.
3. Lambs receiving hydroxyzine in the creep feed manifested significantly higher killing-out percentages and lower thyroid weights than the controls. Pituitary weights were not affected by hydroxyzine.
4. In an experiment on individually-penned, weaned wethers, given a high energy diet, hydroxyzine by either route did not affect live-weight gain but significantly increased killing-out percentage, rate of gain of carcass weight and fat deposition in certain tissues. Hydroxyzine did not affect the fresh weights of the pituitary, thyroid and adrenal glands or the weights of the teats.