An experiment was conducted to examine the potential interaction of caffeine and psychological stress in human subjects. More specifically, subjects comprised of those who showed 'high' and 'low' trait anxiety levels as determined by administration of the Spielberger Trait - Anxiety Inventory. Each student consumed caffeine on a regular basis (2 or more cups of coffee and/or tea per day). Subjects were run in a single-blind, randomized crossover design with caffeine (400mg) and placebo. Each subject participated in two, one and a half hour experimental sessions held one week apart. The cardiovascular effects of a high (400mg) dose of caffeine, equivalent to 4 or 5 cups of coffee, were measured during periods of rest and psychological stress. The stressor involved the administration of a mental arithmetic task (serial subtraction) with a set time limit. Subjects were required to perform as well as they could challenging conditions. In addition, subjects "alertness" and "tension" after each part under reported of each subjective session, using 10cm visual analogue scales representing each of these two dimensions. Previous investigations have documented correlations between caffeine intake and variables of heart rate, blood pressure, alertness, tension, and performance, but there has been no experimental work comparing the response of high and low trait anxiety subjects with respect to these variables. Since caffeine is a central nervous system stimulant it is reasonable to suggest that the drug could be used as a means for low anxiety people to reach an optimal level of arousal, and hence perform optimally in a stressful situation (eg. exams or public speaking). Likewise, it is equally reasonable to suggest that caffeine usage may well be avoided by those people who have high anxiety levels if they are to perform well. The experimental results showed that the order of drug presentation proved more than caffeine administration to be the most recurrent and important variable involved in interaction. Caffeine did not appear to have a potential additive effect on the cardiovascular measurements and subjective self reports, with that produced by psychological stress. High anxiety female subjects showed the greatest elevation in resting systolic and diastolic blood pressure with caffeine consumption. Low anxiety males showed the greatest elevation in diastolic blood pressure with caffeine consumption during the stressful situation. High anxiety males and low anxiety females showed the greatest elevations in heart rate during rest. Males showed greater reported levels of tension with caffeine consumption both at rest and during stress, than did females. High anxiety subjects showed greater elevations in alertness both at rest and during stress following caffeine consumption than low anxiety subjects. Finally, caffeine consumption was found to have no effect on performance in a mental arithmetic task, in both high and low anxiety groups. It is concluded from the results that caffeine administration (400mg) cannot be used as a means for a low anxiety person to reach an optimal drive or stress level to perform optimally during a stressful situation. Likewise, there was no evidence to suggest that those who have high anxiety levels should avoid caffeine consumption to achieve such a goal.