The present study was undertaken to elicit the temporal sequence of changes in cardiovascular and respiratory function during aestivation. Twelve lungfish (2-6 kg) equipped with ECG electrodes, arterial and buccal cannulae, were studied while aestivating in mud or in artificial cloth-bag nests. The periods of observation ranged from 0.5 to 9.5 months. The mean arterial blood pressure gradually decreased from control values of 20-28 mm Hg to a range of 14-18 mmHg during the first 30 days of aestivation, whereas the heart rate dropped more gradually (22-30 beats/min to 11-16 beats/min in 6o days). Ventilatory frequency increased 2- to 5-fold during the first 30 days of encystment and then returned to the control range (2-10 h) within 45 days. The arterial PCOCO2 increased from control values of 25-30 mm Hg to 45-70 mmHg; arterial pH decreased concomitantly from 7.55-7.60 to 7.40-7.26 after the cocoon was formed. The arterial POO2 increased from the control range of 25-40 to 50-58 mmHg during the first 10 days and then returned to the control range. Therefore, the sequential cardiopulmonary changes during the onset of aestivation are gradual and do not parallel the decline in oxygen consumption. Aestivating lungfish also respond promptly to sensory disturbances and thus do not appear to be in a deep torpor. Aestivation is pictured as a state of dormancy, gradual in onset, and the consequence of a complicated physiological interplay.