The small vessel blood content (SVBC) of the ventricular walls of the heart of anesthetized closed chest rats was determined using <sup>59</sup>FeCl<sub>3</sub> to label the plasma siderophilin. SVBC was measured breathing air, 100% O<sub>2’</sub>, 5% CO<sub>2’</sub> in 21% O<sub>2’</sub> and during asphyxia. The average SVBC could be ranked: air < 100% O<sub>2’</sub> < 5% CO<sub>2</sub> < asphyxia. Only the ‘100% O<sub>2</sub>’ values were not significantly above the ‘air’ values. Under control conditions, SVBC of the rat heart averaged 6.77 ± (SE) 0.23 ml blood/100 g tissue. Inhalation of 5% CO<sub>2</sub> increased this to 8.68 ± 0.34, while asphyxia produced a maximal response to 14.40 ± 0.77. Comparison of SVBC were made between various regions of the ventricular walls. The only significant difference was that the right side of the septum had a lower SVBC than the left ventricular sub-epicardium under all conditions except CO<sub>2</sub>. These differences in SVBC are related to the number of open capillaries in the myocardium. Thus inhalation of CO<sub>2</sub> decreases intercapillary distance while 100% O<sub>2</sub> has no effect.