This paper provides a detailed report on measurement of AChE and GST activities in D. magna. We find the rinsing of the excess chemicals from the surface of the animals and homogenizer an important precaution to exclude the possible in vitro effect of the chemicals on enzymes after sample preparation. Such effects were implied in our preliminary experiments with no rinsing, where a clear inhibition of AChE activity was noticed after exposure of daphnids to Cr6+, Cd2+, and diazinon. The methods for measuring AChE and GST activities in D. magna exposed in a standard acute toxicity test are demanding, time-consuming and hence not well suited to routine use. A large number of neonates (up to 800) of the same age and derived from the second to fifth broods are needed in these tests. We recommend the use of these two enzymes when the mode of action of chemicals and daphnids' response to them are under study and we also suggest further testing of these enzymes after longer exposure periods. Our current experiments show that this approach leads to more pronounced changes in AChE and GST activities and, as the animals are larger, fewer of them are needed (Jemec et al., unpublished).