Most radionuclide methods for measuring cardiac volume require a determination of the blood radioactivity concentration. Thus, changes in blood radioactivity over time or during interventions might lead to spurious volume estimates unless blood radioactivity is serially measured. The effects of elevated epinephrine, posture and exercise on 99mTc-labeled blood radioactivity concentration were studied in 15 young (mean age = 28 yr) and 14 older (mean age = 68 yr) healthy males. An epinephrine infusion of 50 ng/kg/min resulted in a 4.1% +/- 1.0% increase in 99mTc-blood radioactivity (p less than or equal to 0.001) compared to baseline. Sitting increased blood radioactivity concentration by 12.3% +/- 3.0% (p less than 0.0002) compared to the supine position and peak supine bicycle exercise caused an 11.0% +/- 1.7% increase (p less than or equal to 0.0001) compared to supine rest. There was a significantly greater increase during peak supine exercise in the young compared to the older subjects (15.0% +/- 2.3% versus 6.3% +/- 2.0%, p less than or equal to 0.01). The mechanism of the increase in blood radioactivity concentration is uncertain, but presumably reflects the addition of hemoconcentrated red blood cells from the spleen and/or the loss of plasma volume. Failure to correct for the increased blood radioactivity concentration during exercise or pharmacological interventions will result in a significant error in serial measurements of cardiac volumes by methods requiring RBC radioactivity measurements.