Free access of 0.5% Guan-mu-tong (Gmt) as drinking water inhibited lactation of mice estimated by pup growth rate, which was associated with a decrease in RNA content and RNA/DNA ratio in the mammary glands. 0.05% Gmt improved pup growth rate and mammary DNA content. No pups were lost during the experiment. Similar results were obtained by 0.5% Xia-ru-yong-quan-tang (Xryqt), a Chinese herbal medicine, which contained 0.05% Gmt. Xryqt further prevented a decline in ovarian weight seen by 0.05% Gmt treatment and significantly increased mother weight. These findings revealed two opposite effects of Gmt on mammary glands, stimulative and inhibitory, at low- and high-dose levels, respectively.