The responses of larval fish to toxicants in subchronic toxicity tests may be modified by events and stressors not associated with the toxicants themselves. In the following work, three potential modifying factors related to larval fish growth, survival, and response to toxicant stress were investigated. Using four-day acute cadmium toxicity tests and seven-day subchronic toxicity tests, differences in the responses of larvae based on the age of the larvae, their exposure to methylene blue, and changes in environmental pH were determined. The age of larvae chosen for toxicity tests, 1 to 24 h old, 24 to 48 h, or 48 to 72 h, did not significantly affect the cadmium (Cd) LC_50S in acute tests or the larval growth or survival in subchronic tests. Prior methylene blue (MB) exposure in larval fish was. not found to affect the Cd LC₅₀ compared to unexposed larvae. However, in subchronic tests with industrial effluents, prior MB exposure resulted in lowered survival or lowered growth, but did not increase sensitivity to the effluent. Lowered growth and survival was observed in blue- dyed individuals from larval populations exposed to MB during the interval from hatch to test commencement. At 20°C, the acute toxicity of MB, as indicated by the LC_10S, was apparent at concentrations greater than 44 mg MB/L in 24 h. At the suggested fathead minnow culture temperature, 25°C, concentrations greater than 5.0 ppm were acutely toxic in 96 h. Chronic toxicity, determined at 25°C, was found at levels greater than 2.12 mg MB/L. Light intensity negatively affected MB chronic toxicity. Chronic values estimated by larval fish growth response were significantly lower in the test done under normal lab light compared to that done under very subdued light. Larval fathead minnows were sensitive to low pH stress based on the acid used to lower test solution pH. Hydrochloric acid was found least toxic to larvae at low PH; phosphoric acid was found most toxic. Larvae tolerated instantaneous changes in pH 2.0 units lower and 1.5 units higher than their culture water without any significant decrease in growth and survival as measured in 7 d. These results indicate that the pH tolerance range for larval fathead minnows, where no effects are seen on growth or survival, is greater than pH 6.0 and less than pH 9.5. / Master of Science