The effect of histamine on the uterine motility was investigated in the nonpregnant adult rabbits in vitro and in vivo. Arterial blood pressure and intrauterine pressure, which was l;\enerateq by uterine contraction and transmitted via liquid column in a polyethylene tubing inserted in the uterine cavity. were recorded on a physiograph through the pressure transducer before and during histamine infusion. The longitudinal tension of the isolated uterine muscle strip was recorded, too, by way of force transducer through cotton thread The uterine motility was represented by the mag• nitude of impulse which was obtained from the area beneath pressure or tension curve and the frequency of the uterine contraction which was counted by the number of waves on recording paper in a finite period. The results obtained were as follows: 1. Arterial blood pressure was dropped by the infusion of 100 μg/ml histamine solution. 2. The uterine motility was increased by the administration of histamine in vivo. 3. The motility of the uterine muscle strip was remarkably increased in 0.5μg/ml histamine solution and it returned to its original level within several minutes after returning to Ringer-Tyrode’s solution containing no histamine. It was inferred that histamine might stimulate uterine smooth muscle of nonpregnant adult rabbit by its direct action.