The cardiac index was measured in 100 normal men, 20 in each decade from 40 to 89 yrs., and in addition in 2 men of 91, and 1 of 101 yrs. The index declines 0.18 liter (7.3%) during this period or in each decade 0.35 liter (1.5%). The correlation is negative and is [long dash]0.1862 [plus or minus] 0.0965. It is not significant. If the entire series of 103 men is utilized it is [long dash]02039 [plus or minus] 0.0944 and is significant. The rate of decline is almost the same. The mean values in succeeding decades or half-decades suggest that the rate of decline of the curve is arrested for a time after 55 years. To smooth the curve more observations are required. Variations in each age group exist as in vounger men. In any 5 or 10 year group the maximum deviations from the respective means were 21.6 and 27.1%. The coefficient of variation of the means varied from 8.3 to 16.6%. For the entire series (40-101 yrs.) the mean is 2.26 liters per sq. m. per min.; the standard deviation is 0.26 (11.6% of the mean) and the standard error of the mean is 0.026. For the evenly distributed group of 100 men the results are unchanged; the mean value was 2.26 liters, the standard deviation 0.26 (11.5% of the mean), the standard error of the mean 0.026, and the standard error of estimate 0.259 liter. The decline in cardiac index follows chiefly upon the decline of the O2 consumption since the influence of change in arterio-venous oxygen difference is small. Though the measurement of the index indicates slight decrease in the work of the heart under basal conditions, there remain to be considered other functions which are involved in changes in cardiac activity with age.