We report research on different phasic evoked cardiac responses associated with differences in cogintive activity.These were examined in relation to a stable individual difference variable, mean simple reaction time (RT). Individual means on RT were found to be sufficiently stable over a 10 month period to consider them as individual functional characteristics. Subjects were divided into two subgroups on the basis of the first measure of their individual mean RT (above and below the group median). Each subject received 10 innocuous auditory stimuli with randomly varying interstimulus intervals. Stimuli were presented in one of two conditions defined by instructions allowing them to ignore (irrelevant condition), or requiring them to count the stimuli (relevant condition). A main effect of instruction was obtained in the evoked cardiac response. The initial heart rate deceleration was significantly larger in the relevant condition. Short-RT subjects had smaller heart rate changes to the irrelevant stimuli. The data are discussed in terms of the intensity of stimulus processing (both physical and cognitive) as a factor which may be related fundamentally to stable individual differences in RT.