DICKER, SCOTT C., GERALDINE K. LOFTHUS, NORTON W. THORNTON, and GEORGE A. BROOKS. Respiratory and heart rate responses to tethered controlled frequency breathing swimming. Med. Sci. Sports Exercise, Vol. 12, No 1, pp. 20–23, 1980. The purpose of this study was to investigate the respiratory and heart rate (fH) responses to tethered controlled frequency breathing (CFB) swimming. Controlled frequency breathing swimming is an aquatic training technique in which ventilatory rate is voluntarily reduced in order to induce systemic hypoxia during training. Nine elite college swimmers experienced with CFB were studied. The tethered swimming tests were discontinuous, with 4 min work bouts interspersed with equal duration rest periods. The resisting forces during tethered swimming were 5.63, 6.82, and 7.95 kg. Each subject was tested breathing every two (control), three, four, and five arm strokes. Subjects performed all four breathing frequencies at a constant arm stroke rate of 30/min during freestyle swimming. As ventilatory volume decreased due to CFB, O2 extraction and estimated tidal volume significantly increased (P < .05) to maintain a constant O2 consumption for a given workload. Carbon dioxide production, respiratory exchange ratio, and fH did not change significantly in response to CFB. Estimated alveolar partial pressure of O2 (Pao2) decreased and Pao2 increased significantly during CFB. However, estimated saturation of arterial blood with O2 (Sao2) was essentially undiminished during CFB. These responses do not indicate hypoxia, but rather hypereapnia during CFB.