Summary Oscars are often subjected to low levels of oxygen and fasting during nest-guarding on Amazonian floodplains. We questioned if this anorexia would compromise ionoregulation under the hypoxic, ion-poor Amazonian conditions. We compared fed and fasted oscars (10-14 days), in both normoxia and hypoxia (10-20 Torr, 4-8 hours). Routine oxygen consumption rates (MO2) were lower in fed fish, reflecting behavioural differences, but fasting improved hypoxia resistance with lower critical oxygen tensions (Pcrit). Energy stores and activities of enzymes related to energy metabolism in liver, muscle or gills were not affected. However, hepatosomatic index was reduced by more than 50% in fasted fish, indicating lower total energy stores. Branchial Na+ uptake rates were lower in fed fish, whereas Na+ efflux was similar. Fed and fasted fish quickly reduced Na+ uptake and efflux during hypoxia, with fasting fish responding more rapidly. Ammonia excretion and K+ efflux were reduced under hypoxia, indicating decreased transcellular permeability. Fasted fish had more mitochondria rich cells (MRC), with larger crypts. Gill MRC density and surface area were greatly reduced under hypoxia, possibly to reduce ion uptake and efflux rates. Density of mucous cells of normoxic fasted fish was about four-fold of that in fed fish. This was reduced to half in animals exposed to hypoxia, with clear mucous deposits on the epithelial surface. Overall, a 10-14 day fasting period had no negative effects on hypoxia tolerance in oscars, as fasted fish were able to respond quicker to lower oxygen levels and turned down branchial permeability effectively.