Characteristics of uterine activity produced by nipple stimulation were studied in 185 consecutive breast stimulation stress tests. Adequate contractions were produced in 95.6% of tests. Exaggerated uterine response (contractions occurring more than once every two minutes or lasting more than 90 seconds) was present in 45.5% of the patients. Twenty-one percent of the patients with such uterine activity also had a fetal heart rate (FHR) deceleration (hyperstimulation breast stimulation stress test), without adverse fetal outcome. The time in minutes from start of nipple stimulation to the first contraction (stimulation contraction interval) was recorded for each patient. Significant difference was not observed in the stimulation contraction interval distribution between the groups with and without exaggerated uterine activity. The authors conclude that there is a relatively high incidence of exaggerated uterine activity response to the breast stimulation stress test and that close surveillance of mother and fetus is warranted during antepartum nipple stimulation.