Nitrite that accumulates during intensive culture of channel catfish (Ictalurus punctatus) often results in high mortality. This study evaluated sublethal effects of nitrite on young channel catfish. Fish (7–10 cm standard length) exposed to 1, 2, 3, 4, or 5 mg/liter of nitrite for 24 hours developed mean methemoglobin concentrations (% of total hemoglobin) of 35, 79, 79, 85, and 90%, respectively. During the 5 mg/liter exposure, methemoglobin increased rapidly during the first 6 hours from 7% to 59%, more slowly to about 80% by 12 hours, and trivially thereafter. Channel catfish exposed to 5 mg/liter nitrite for 6 hours and transferred to nitrite-free water had near normal amounts of methemoglobin (13%) after 24 hours. An inverse relationship was noted between pH of aquarium water and methemoglobin formation after exposure to nitrite. Addition of calcium chloride, potassium chloride, sodium chloride, and sodium bicarbonate to the water suppressed methemoglobin formation in fish exposed to nitrite, but sodium sulfate had no such protective effect.