Quantitative bioenergetic and physiological measurements were made on juvenile rainbow troutOncorhynchus mykissexposed over summer (June–September 1993) to a simulated summer warming scenario of +2°C in the presence and absence of 70 μmol total ammonia/L (nominal; equivalent to 0.013 mg NH3-N/L at 15°C, pH = 7.6) to determine the metabolic costs and physiological consequences associated with their growth in a warmer, more polluted environment. With unlimited food, fish exposed to +2°C show better energy conversion efficiency and increased nitrogen retention at a metabolic cost equivalent to the base temperature group. Metabolic fuel use appears to have been optimized to support the bioenergetic demands imposed during maximum summer water temperatures. Low-level ammonia enhances nitrogen and energy conversion efficiency by stimulating protein retention, which ultimately results in the most cost-effective growth. However, in the +2°C ammonia treatment, the stimulatory effect of low-level ammonia is lost during mid to late summer due to the greater energy demands when fish are forced to cope with the additional stress of a small further increase in temperature.