In 16 young house physicians, each of whom presented a case at grand rounds, the effect on platelet function of the emotional stress involved in presenting the case was determined. Platelet-aggregation curves, using adenosine diphosphate and epinephrine, were determined immediately before, immediately after, and seven days after stress. Platelet aggregation was diminished during the stress period in 15 of the subjects. These findings may possibly be of importance in the relationship between stress and myocardial infarction; they also suggest that the emotional state of the patient must be considered when interpreting platelet-aggregation curves.