Postpartum bleeding of the uterus is an important cause of maternal death. It is thought that the inhibitory effect of the majority of potent inhalation anesthetics on uterine contraction is often responsible (Munson, 1970; Lim et al., 1971; Kim, 1972). There have been numerous reports that ketamine stimulates uterine contraction clinically Chodoff and Stella, 1966; Little et al, 1972; Galbert and Gardner, 1973; Corssen, 1974). Jawalekar and associates(1972) reported that ketamine increased resting tension, contractile amplitude and frequency of the uterine strips of pregnant mice. And Kim(1975) reported research on the effects of ketamine on the isolated uterus of rabbits. According to Kim(1975), ketamine exerted .a stimulatory action on the uterus under the influence of progesterone. This progesterone-depenent uterine stimulatory action of ketamine is not concerned with adrenergic and cholinergic mechanisms but appears to have a direct effect on the uterine muscle. The present study was undertaken to determine whether or not ketamine also exerted a stimlatory action on the uterus of the guinea pig and to follow up whether or not stimulatory action of ketamine also depends upon progesterone in the guinea pig. Adult female non-pregnant guinea pigs and rabbits weighing approximately 0, 5 kg and 2. 0 kg respectively were employed in this experiment. At the end of 10 to 14 days following bilateral oophorectomy, non-pregnant guinea pigs and rabbits were injected intramusculary with estradiol benzoate (2,000 I.U./kg) or progesterone (Smg/kg) once a day for four consecutive days, A uterine strip, about 1.5~2,0cm in length, was carefully isolated from the experimental animals and suspended in a muscle chamber containing 50 ml of Lockes solution, maintained at constant temperature of 38C. It was aerated with 100% oxygen bubbling through the bathing fluid by means of a sintered glass plate at the bottom of the muscle chamber. One end of the uterine strip was attached to the bottom of the muscle chamber and the other end to a lever. Motility and tonus were recorded on kymograph paper. After being washed several times with fresh Locke's solution, the uterine strip attained a constant motility and tonus. Ketamine then was added in various concentrations to the chamber. The results are as follows: 1. The uterine mctility of guinea pig shows a higher amplitude and lower frequency compared with that of rabbit. 2. Effects of ketamine on isolated uterine strips of guinea pig pre-treated with estrogen showed no change using 1~20 γ/ml of ketamine, but with 50 γ/ml or more, the amplitude of uterine contractions was depressed. 3. On isolated uterine strips of guinea pig pre-treated with progesterone, there was no effect with 1~100 γ/ml of ketamine, depressed amplitude of uterine contractions with 150 γ/ml and markedly depressed amplitude of uterine contraction with 200 γ/ml. 4. On isolated uterine strips of rabbit pre-treated with estrogen, there was no effect with 20 γ/ml of ketamine, but depressed amplitude of uterine contraction using 50 γ/ml. 5. On isolated uterine strips of rabbit pretreated with progesterone, there was an increase in amplitude and frequency of uterine contractions with up to 150 γ/ml of ketamine, but depressed amplitude with 200 γ/ml or more. From the above results it may be concluded that ketamine exerted a stimulatory action on the uterus of rabbit under the influence of progesterone but not on the uterus of guinea pig. The large amount of ketamine depressed the amplitude of uterine contraction in guinea pig and rabbit and this depressing dose is higher in the animals (guinea pig and rabbit) pre-treated with pr ogesterone than with estrogen.