There are a number of reports concerning the physiological effects of voluntary hyperventilation in the human.1-7Hyperventilation has been said to be an extremely fatiguing activity. Accordingly, most of the previous studies on voluntary hyperventilators have been for relatively short periods of time. The purpose of this report is to point out our observations in 4 volunteers who hyperventilated for periods up to 4 hours and longer. Observations were made regarding (1) subjective symptoms and (2) changes in electrolytes. The fall of serum inorganic phosphorus concentration was so marked in our cases that we felt justified in reemphasizing this observation. Procedure By means of the Beckman Spinco infrared carbon dioxide analyzer, the alveolar CO2was monitored via an intranasal catheter and maintained in the range of 2.0% to 2.5% (or approximately one-half the normal level for these subjects). The rate and amplitude of respiration were constantly adjusted to