Experimenter effects have long been hypothesized to influence participants' responses to mental stress testing. To explore the influence of experimenter warmth on responses to two mental stress tasks (mental arithmetic, mirror tracing), 32 young women participated in a single 45-min experimental session. Participants were randomized into warm experimenter or cold experimenter conditions that were operationalized using differential contact scripts, interpersonal styles, and attire. Heart rate (HR) and systolic and diastolic blood pressure (SBP and DBP) were measured during rest and task periods, and participants completed subjective ratings of anxiety and several items rating the experimenter. Results showed greater effort and improved performance on both tasks for participants in the warm experimenter condition, although no significant differences in HR, SBP, DBP, or subjective ratings of anxiety were observed between the conditions. These findings suggest that experimenter effects have a more robust influence on motivational factors than on physiologic or emotional response parameters. (Contains 4 tables.)