The contraction stress test (CST) was used to attempt determination of fetal reserve prior to labor by evaluating the response of the fetal heart rate to spontaneous or induced uterine contractions. Testing was performed by using an external fetal heart rate monitor and tocograph. The CST was employed 189 times in 120 high-risk patients who either had hypertensive disorder of pregnancy or had completed at least 42 weeks of gestation. Testing was begun as early as 34 weeks' gestation and repeated at weekly intervals; it was not used in the clinical management of the patient. The maximum number of tests performed on one patient was 8. A negative CST appears to be a most reliable guide to the ability of the fetus to tolerate labor if it ensues within 1 week. A positive test suggests that the fetus is at increased risk, but death is not necessarily imminent. Results of the present study warrant use of the CST in controlled studies to determine its effect in improving perinatal outcome.