Increased hemoglobin concentration (Hb) and hematocrit (Hct), attributable to spleen contraction, raise blood gas storage capacity during apnea, but the mechanisms that trigger this response have not been clarified. We focused on the role of hypoxia in triggering these Hb and Hct elevations. After horizontal rest for 20 min, 10 volunteers performed 3 maximal apneas spaced by 2 min, each preceded by a deep inspiration of air. The series was repeated using the same apneic durations but after 1 min of 100% oxygen (O(2)) breathing and O(2) inspiration prior to each apnea. Mean apneic durations were 150, 171, and 214 s for apneas 1, 2, and 3, respectively. Relative to pre-apnea values, the mean post-apneic arterial O(2) saturation nadir was 84.7% after the air trial and 98% after the O(2) trial. A more pronounced elevation of both Hb and Hct occurred during the air trial: after apnea 1 with air, mean Hb had increased by 1.5% (P < 0.01), but no clear increase was found after the first apnea with O(2). After the third apnea with air Hb had increased by 3.0% (P < 0.01), and with O(2) by 2.0% (P < 0.01). After the first apnea with air Hct had increased by 1.9% (P < 0.01) and after 3 apneas by 3.0% (P < 0.01), but Hct did not change significantly in the O(2) trial. In both trials, Hb and Hct were at pre-apneic levels 10 min after apneas. Diving bradycardia during apnea was the same in both trials. We conclude that hypoxia contributes to spleen contraction during apnea, likely through chemosensor-related sympathetic output. There are, however other factors involved that trigger spleen contraction even in the absence of hypoxia.