Abstract: Toxic effects of chlorpyrifos and endosulfan on Macrobrachium nipponense, a kind of freshwater shrimps, were investigated for the first time. Firstly, acute toxicity tests of chlorpyrifos and endosulfan were conducted, separately, to determine 24 and 48h ρ(LC₅₀) of the insecticides to the shrimps. In the tests, six treatments were set up within the range of 0-48h ρ(LC₅₀), and the shrimps were kept exposed for 24h for determination of their oxygen consumption and ammonia excretion. It was found that the 24 and 48h ρ(LC₅₀) was 5.03  and 2.01µg•L^-1, respectively, for chlorpyrifos and 8.47 and 4.02µg•L^-1, respectively, for endosulfan, and their safe concentration was 0.096  and 0.272µg•L^-1, respectively, suggesting that the shrimps are more sensitive to chlorpyrifos than to endosulfan. With the increasing concentrations of the two insecticides, both the oxygen consumption and ammonia excretion of the shrimps decreased in volume, indicating that the shrimps use the same mode of adaptability to protect their basic metabolism against the stress of the two insecticides, that is to say, passive endurance.

Key words: chlorpyrifos, endosulfan, Macrobrachium nipponense, LC₅₀, 24 hours stress test