A 6 x 3 factorial laboratory experiment was conducted to study the effects of acid and alkaline stress (pH 5.0, 6.0, 7.0, 8.0, 9.0, and 10.0) on the oxygen consumption rate (OR), ammonia excretion rate (NR), and lactate dehydrogenase (LDH) and Na+ -K+ adenosine triphosphatase (Na+ -K+ ATPase) activities of Oreochromis niloticus juveniles with body mass 1.02, 5.13, and 10.31 g. With increasing pH, the juveniles OR and NR increased first, peaked at pH 7.0-8.0, and decreased then. The OR and NR increased with increasing body mass, and their relationships fitted power equations. The linear effects of pH and body mass and the quadratic effect of pH on the OR and NR were highly significant (P < 0.01), but the pH and body mass had less synergistic effect on the OR and NR (P > 0.05). Regressive equations of pH and body mass with the OR and NR were established, the R2 being 0.942 and 0.936, respectively (P < 0.01). Body mass had significant effects on the O:N ratio (P < 0.01), whereas acid and alkaline stress could alter the energy source utilization patterns of the juveniles. High pH was not favorable to the LDH activity, but favorable to the Na+ -K+ ATPase activity. The pH had linear and quadratic effects on the LDH and Na+ -K+ ATPase activities (P < 0.01), body mass had no significant effect on the Na+ -K+ ATPase activity, and the pH and body mass had no synergistic effect on the two enzyme activities (P > 0.05).