Physiological aspects like heat balance, gas exchange, osmoregulation, anddigestion of the early Permian aquatic temnospondyl Archegosaurusdecheni, which lived in a tropical freshwater lake, are assessed basedon osteological correlates of physiologically relevant soft-tissue organsand by physiological estimations analogous to air-breathing fishes. Bodymass (M) of an adult Archegosaurus with an overall body length ofmore than 1 m is estimated as 7 kg using graphic double integration.Standard metabolic rate (SMR) at 20 °C (12 kJ h⁻¹) and activemetabolic rate (AMR) at 25 °C (47 kJ h⁻¹) were estimated accordingto the interspecific allometry of metabolic rate (measured as oxygenconsumption) of all fish (V_O2 = 4. 8 M^0. 88) and form the basis formost of the subsequent estimations. Archegosaurus is interpreted asa facultative air breather that got O₂ from the internal gills at restin well-aerated water but relied on its lungs for O₂ uptake in timesof activity and hypoxia. The bulk of CO₂ was always eliminated via thegills. Our estimations suggest that if Archegosaurus did not havegills and released 100 % CO₂ from its lungs, it would have to breathemuch more frequently to release enough CO₂ relative to the lungventilation required for just O₂ uptake. Estimations of absorption andassimilation in the digestive tract of Archegosaurus suggest thatan adult had to eat about six middle-sized specimens of the acanthodian fishAcanthodes (ca. 8 cm body length) per day to meet its energydemands. Archegosaurus is regarded as an ammonotelic animal thatexcreted ammonia (NH₃) directly to the water through the gills and theskin, and these diffusional routes dominated nitrogen excretion by thekidneys as urine. Osmotic influx of water through the gills had to becompensated for by production of dilute, hypoosmotic urine by the kidneys.Whereas Archegosaurus has long been regarded as a salamander-like animal, there is evidence that its physiology was more fish-than tetrapod-like in many respects.