ABSTRACT: Rates of oxygen consumption (R) and ammonia excretion (E) of 29 species of small teleost fishes, which weighed between 1 and 400 mg dry mass (DM), from inshore waters of the Great Barrier Reef were determined at in situ temperatures (25 to 30°C). Regression analyses revealed that R (6.7 to 1296 µl O₂ ind.^–1 h^–1) and E (0.28 to 64.2 µg NH₄-N ind.^–1 h^–1) were correlated with body mass, but the ratio of R to E ( oxygen to nitrogen ratio; 17 to 104 by atoms), was not. Water content of fish bodies ranged from 66.0 to 81.4% of wet mass (WM), and ash content from 11.9 to 28.6% of DM. Total carbon (C) and total nitrogen (N) composition varied from 36.2 to 44.4% and from 8.3 to 12.8% of DM, respectively, resulting in C:N ratios of 3.1 to 4.7. Fractions of inorganic C and N were small (0.04 to 0.33% and 0.01 to 0.15% of DM, respectively). Combining R and E data with those of body C and N composition, daily metabolic losses were estimated to be 4.3 to 18.6% for body C and 0.8 to 9.1% for body N. The present R–body mass relationships were compared with the 3 published predictive models for fishes to explore the best fit model. On a body mass basis expressed by N, values for R were consistent with the model for epipelagic zooplankton, but values for E were 30% lower, suggesting somewhat reduced E relative to R in fishes as compared with zooplankton. Three out of the 29 fishes exhibited markedly high metabolic O:N ratios together with high body C:N ratios, which was interpreted as an adaptation to N-limited detritus nutrition.