To improve survival of stocked fish, field and laboratory tests were conducted to evaluate the survival and stress response of rainbow troutOncorhynchus mykissafter exposure to waters with various combinations of high temperature and high pH. For each of four laboratory experiments, fish were transported by truck for 90 min. Fish were then put in replicate tanks for each of four treatments per experiment: (A) controls, pH 7.8, temperature 14°C; (B) control pH and high temperatures (19°C, experiment 2; 22°C, experiments 1, and 4) or low temperatures (7–9°C, experiment 3); (C) control temperature and fluctuating high pH (8.4–9.6); and (D) a combination of high or low temperature with high pH. Blood was sampled at 1.5, 3, 6, and 24 h after stocking. At 1.5 h, plasma glucose, chloride, and cortisol levels in all fish indicated a stress response from hauling and stocking. High temperatures (19°C, 22°C) alone did not produce additional changes in the stress indicators. However, high pH induced significant additional rises in glucose and cortisol levels in both high- and control-temperature tanks. At low temperatures the stress response was delayed; low temperature (7–9°C) alone produced significant elevations in glucose and cortisol compared with controls at 24 h, suggesting that cold water acted as a stressor, albeit with a delayed reaction. Cold water combined with high pH induced higher cortisol concentrations after 24 h than high pH alone. Warm temperatures combined with high pH did not synergistically affect the stress response, but they significantly increased mortality at 22°C. Mortality in the field occurred at pH levels greater than 9.3–9.4 and temperatures of 19.9–22.8°C. Diel fluctuations in pH measured in four reservoirs ranged 0.1–0.5 units. Laboratory and field tests indicated that pH values greater than 9.4 resulted in mortality, especially at higher temperatures. There was also a significant stress response to pH 9.0 or greater.