This study of the goldfish (Carassius auratus L.) combines two techniques: heat measurements via direct calorimetry and radio telemetry, using small implantable telemetry transmitters (3 g). These record overall metabolic rate, and electrocardiogram (ECG) and heart rate frequency (fHR), respectively. The metabolic rate decreased at hypoxia levels of 40. 20 10, and 3% air-saturation (AS) almost linearly to 94, 84, 69, and 55% of the standard metabolic rate (SMR), respectively. This implies that metabolic depression is flexible, depending on the Supply of oxygen. From the deconvoluted heat-flow signal it can be concluded that the metabolic depression per hypoxia level takes place within 20 min. At 3% AS anaerobic metabolism was strongly activated. The fHR of 34 beats per minute (bpm) at normoxia fell at hypoxia levels of 40, 20, 10, and 3% AS to 26. 22. 14, and 9 bpm, respectively. A correlation coefficient of 0.97 was calculated between the level of metabolic depression and decrease of fHR suggesting a relationship between level of metabolic depression and the HR. These results support the hypothesis that blood flow reduction is the proximate cause for the observed metabolic depression. (C) 2003 Elsevier B.V. All rights reserved.