This study examined the effect of introducing either a male or female observer on the ratings of perceived exertion (RPE) and affect of male runners during a moderate intensity running task. 10 moderately active men completed three 20-min. moderate intensity running trials at 60% of their peak treadmill running speed. Each participant completed three trials in random order: control, male-observed, and female-observed, where either the male or female observer joined the trial after 10 min. of the trial had elapsed, during which RPE and affect were monitored. The introduction of a female observer caused a significant decrease in RPE, whereas the introduction of a male observer caused a significant increase in RPE compared to the control trial. Affect was higher in the presence of both a male and female observer compared to control. It was concluded that there is a social, interpersonal, psychological dimension to RPE during exercise.