1. Ventilation and cardiac frequency were measured during repeated treadmill exercise in three healthy subjects over 36 weeks, before, during and after iron-deficiency anaemia was produced and after iron treatment. The haemoglobin and 2,3-diphosphoglycerate concentrations and the oxygen-binding (P50) were measured.
2. There was an inverse relationship between the haemoglobin concentrations and the 2,3-diphosphoglycerate concentrations and the P50 values.
3. The mean cardiac frequencies during the fourth to tenth minutes of exercise showed a negative correlation with the haemoglobin concentrations in all three subjects, and the mean minute ventilations in two of them.