The sublethal effects of methyl parathion, sodium nitrite as well as thecombination of the two toxic substances on the level of cytochrome P450, cytochromeP420, cytochrome b5 and cytochrome C reductase as well as changing in liver weightand microsomal protein content were studied in crossbred catfishes. The catfisheswere exposed to methyl parathion ( 0.1-4.0 mg/L ), sodium nitrite ( 0.625-150 mg/L)and the combination of methyl parathion ( 0.5 and 1.0 mg/L ) and sodium nitrite(25-150 mg/L) for 24 hours. The results from these studies showed that methylparathion at the concentration up to 2 mg/L caused a significant decrease of thecytochrome P450 level accompanied with an elevation of an absorption peak at 420which supposed to be cytochrome P420, a denatured form of cytochrome P450. Thereduction of cytochrome P450 with elevation of cytochrome P420 were found to beconcentration dependent. No significant changes in cytochrome b5, liver weight andmicrosomal protein content were found. At the concentration up to 1 mg/L, methylparathion caused a significant decrease in cytochrome C reductase activity. Sodium nitrite caused also a concentration dependent decresed in cytochromeP450 with increasing in cytochrome P420 level. At concentration up to 100 mg/L,sodium nitrite caused significant decrease in cytochrome P450 (P<0.01) withincreasing formation of cytochrome P420 (P<0.001). Significant decrease (P<0.001) ofcytochrome C reductase was showed at the high concentration of sodium nitrite((+,>)25 mg/L). No significant changes of cytochrome b5, microsomal protein and liverweight were also observed. Combining methyl parathion and sodium nitrite produced a less than additiveeffect on these measurements. Some of these effects seen might have implication formonitoring of environmental pollution for these two toxic compounds.