Zilpaterol is a β-adrenergic receptor agonist. β-adrenergic receptors, prototypic G-protein-coupled receptors, play a key role in regulating numerous physiological processes. In vitro: Previus study found that there was no effect of zilpaterol (ZH) on thymidine incorporation into DNA. ZH at 1 μM could decrease β1-AR mRNA, and 0.01 and 1 μM ZH decreased β2-AR and β3-AR mRNA in myoblasts. There was no effect of ZH on the β-AR or IGF-I gene expression in fused myotube cultures. The β2-AR antagonist ICI-118, 551 attenuated the effect of ZH to reduce expression of β1- and β2-AR mRNA. There was no effect of ICI-118, 551 or ZH on β3-AR or IGF-I. In addition, the total lipid synthesis from acetate was increased by ZH in bovine subcutaneous adipose tissue explants in the absence of theophylline . In vivo: FDA has approved label claims of increased rate of BW gain, improved feed efficiency, and increased carcass leanness. The feeding of ZH for 20 to 40 d with a 3-d withdrawal resulted in significantly increased ADG. Such increases equate to an average of 9 kg heavier BW in ZH-fed steers. Moreover, hot carcass weight has been shown to increase to a larger degree compared with BW, with an average improvement of 15 kg . Clinical trial: So far, no clinical study has been conducted.