The pain thresholds of a group of introverts and extraverts were assessed. It was found that the mean pain threshold of the introverts was significantly lower than that of the extraverts. In a further experiment it was found that caffeine, a stimulant drug, lowered the mean pain threshold significantly. The relation between the perception of pain and level of arousal is discussed, and it is argued that the difference in mean pain threshold values as between introverts and extraverts is attributable to a difference in level of arousal.