Although there is little direct evidence available, there is a pervasive assumption that HR is easily detected during periods of physical stress or exertion. It has also been suggested that distance runners are keenly aware of their cardiac functioning. The present study assessed cardiac awareness while at rest and again following vigorous exercise for groups of subjects differing in physical fitness. Thirty-six subjects were solicited and composed groups of Sedentary, Tennis, and Distance Running groups. Each group was composed of equal numbers of male and female subjects. HR awareness was assessed using 35 10-sec discrimination trials. Veridical feedback was composed of light flashes triggered by EKG R-spikes whereas false feedback was composed of flashes at rates 10, 20, or 30 percent above or below their actual rate. At the end of each trial, subjects indicated the degree of certainty of their choice. Discrimination accuracy was measured at rest and again following exercise which raised HR level by 75 percent. Analyses of variance indicated that male runners were significantly more accurate discriminators at rest than any other group. Similar superiority was not shown by the female runners. After exercise, the tennis and sedentary groups showed significant overall increases in awareness during augmented cardiac functioning. Exercise did not produce additional increments in awareness for the male runners. A battery of post experimental questionnaires largely failed to discriminate experimental groups.