Apnea is physiologically stressful and can within a short time frame pose a threat to survival. To sustain prolonged apnea, oxygen use should be minimized and its storage maximized. Two mechanisms known to have this effect are the cardiovascular diving response, directing the available oxygen selectively to the heart and brain, and spleen contraction increasing the circulating erythrocyte volume and thereby gas storage capacity. Spleen contraction is also observed after maximal exercise, and is thought to enhance aerobic performance. While the cardiovascular diving response is known to be more pronounced after apnea training, spleen contraction has not been studied in conjunction with apnea training or other types of training. The aim of the present investigation is to study the hematological responses to apnea performed during rest by elite apneic divers, by elite cross-country skiers and by untrained subjects. After 20 min of rest, subjects performed 3 maximal apneas spaced by 2 min normal breathing intervals. Blood samples were drawn before, directly after, and 10 min after the apnea series and hemoglobin concentration was measured. All groups responded to maximal apneas with an increase in hemoglobin concentration, which had disappeared after 10 min of recovery. The increase in hemoglobin concentration was more pronounced in the apneic divers (4g/L) than in skiers (3g/L) and untrained subjects (2g/L; P < 0.05). All groups prolonged their apneic times through the series, but the increase was most evident for the divers versus both the skiers (P < 0.05) and untrained subjects (P < 0.01). The results suggest that these responses could be more pronounced as a result of apnea training.