The objectives of this study were to examine the effects of β-agonist cimaterol on mammary differentiation and the activity of amino acid uptake in acinar cell culture. Ninety six weanling female rats were assigned to either a control or test (supplementation of cimaterol, 10mg/kg) group. For feeding trial, growth rate and feed intake were examined. Mammary tissues from selected experimental animals were obtained to examine the mammary differentiation by chemical analysis, and to conduct the acinar cell culture for evaluating amino acid uptake activity. Rats on test group had significantly improved the growth rate and feed intake. The supplementation of cimaterol in rat diet increased the ratio of RNA/DNA and protein/DNA, DNA and protein with a decrease in lipid content in mammary glands. Mammary glands from the test group animals had more amino acid uptake activity by acinar cell culture than those from control group animals. These results indicate that supplementation of β-agonist cimaterol in rat diet can influence differentiation and amino acid uptake in mammary gland.