By means of a specially constructed 9-way stopcock, series of immediately successive samples of arterial blood, collected in as short a time as possible, were obtained from patients exhibiting Cheyne-Stokes respiration. Analyses of these samples showed that the minimal O and maximal CO2 concentrations occurred during the earlier part of the respiratory phase, the maximal O and minimal CO2 near the end. Approximately mean concentrations of the gases occurred at the beginning of the respiratory phase. Inhalation of O failed to stop the periodicity and usually resulted in an increase in length of the apneic phase and of the whole cycle, but CO2 in sufficient concentrations invariably brought about continuous respiration by the gradual encroachment of the dyspneic upon the respiratory phase. Study of these phenomena suggests that prolonged circulation time of the blood is not important in bringing about Cheyne-Stokes respiration.