This study was undertaken to illustrate the uterotonic effect of Leonurus sibiricus. It was dissolved in sterile water and several different dosages were administered both in vitro and in vivo study. Rat uterine tissue for in vitro bioassay was obtained from estrous rat. From the low to high dosages of Leonurus sibiricus were tried and each uterine contraction was recorded and integrated. Anesthetized estrous rat for in vivo study was cannulated into the jugular vein for infusion of the compound. Another cannula with a balloon tipped and water filled was inserted into the uterus to measure uterine activity. While the uterine tissue did not respond to low dosage of compound, high dosage of compound stimulated the tissue to contract less than 1 minute with low amplitude. In vivo rat uterus showed a certain, consistent pattern of contractions which was initial relaxation and followed by prolonged and increased amplitude of contractions. It also caused a short breathing stop which might be due to acute acidosis.