The relation between blood pressure and delivery in pregnant rats was investigated when the blood pressure of pregnant rats was controlled by a pressor or antihypertensive drug in the middle or final stage of pregnancy. All of the pregnant rats studied delivered in a 23 degrees C environment in spite of blood pressure control by a pressor or antihypertensive drug. At 33 degrees C, the pregnant rats were divided into two groups whose blood pressure was increased severely (160-175 mm Hg) or moderately (145-155 mm Hg) at 10 days of pregnancy. Those in the former group delivered, but those in the latter died in the final stage of pregnancy. When the pregnant rats in the group with high blood pressure on day 10 were treated with a antihypertensive drug in the middle stage of pregnancy or with a pressor drug in the final stage, the mortality rate was 100 per cent in both cases. The delivery rate was 57 per cent when the pregnant rats were treated with a antihypertensive drug from the middle to the final stage of pregnancy. In the group with low blood pressure on day 10, all of the pregnant rats delivered when they were treated with a pressor drug in the middle stage of pregnancy. However, the delivery rate was 89 per cent when the pregnant rats were treated with antihypertensive drug in the final stage of pregnancy.