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The effect of transferring fish from water at pH 7.0 to either more acid or more alkaline conditions was to reduce the maximum critical velocity of the fish. In water of pH 4.0, 5.0, and 10.0, the maximum critical velocity was only 54.5%, 66.5%, and 61% respectively of that recorded for fish in the water of pH 7.0. Thus, both acid and alkaline conditions in the water reduce the aerobic swimming capacity of trout. Exposure to acid conditions increased mucus secretion and this was associated with an increase in coughing and breathing frequency in resting fish. Coughing rate increased from 41/hr to 592/hr; and respiration frequency increased from 81/min to 104/min when fish were transferred from water at pH 7.0 to water at pH 4.0. In comparing fish exposed to acid and alkaline waters, the results indicates that fish have a greater capacity to regulate blood pH in acid than in alkaline conditions. The gill potential was strongly dependent on water pH, being negative in neutral water, but positive in acid water and more negative in alkaline solution. Catecholamine levels increased significantly during acid exposure, but were not altered during alkaline exposure. The increasing catecholamine levels appeared at different time periods in different fish during acid exposure and seemed to be associated with the death of the fish. Na⁺ and C1⁻ ion concentrations in plasma decreased significantly after 24hrs of acid exposure, but did not change significantly in alkaline water. This may indicate that ionoregulatory disturbance in plasma is one of the reasons for the decrease in the maximum critical velocity in acid water, but not in alkaline water. / Science, Faculty of / Zoology, Department of / Graduate