In this work we investigated the possibility of using the enzyme acetylcholinesterase (AChE) activity in Daphnia magna homogenates, both in vivo and in vitro conditions, as a specific method for rapid toxicity evaluations. The results from in vivo and in vitro AChE inhibition tests were compared with 48 hours EC50 values obtained in conventional acute bioassays. EC50 values from in vivo AChE inhibition tests were: 2.4 micrograms/l for parathion, 0.2 microgram/l for paraoxon; DCA and cadmium at the concentrations tested had no effects on enzyme activity. I50 values were 764 micrograms/l for parathion, 0.08 micrograms/l for paraoxon and 3367 micrograms/l for cadmium; DCA did not affect AChE activity measured in in vitro conditions. EC50 values from conventional acute tests were: 2.2 micrograms/l for parathion, 0.2 microgram/l for paraoxon, 163 micrograms/l for DCA and 9.5 micrograms/l for cadmium. Our results indicated that the in vivo AChE inhibition test is selective, being very sensitive to detect toxicity of the organophosphates tested. The in vitro AChE inhibition assay is less time consuming, requires less human effort and produces less toxic waste than conventional acute bioassays and the in vivo AChE inhibition test. However, it does not take into account the effect of the metabolization of the toxicants inside live organisms; since the organophosphate metabolism may be activative or degradative, the toxic potential of the parent compound may be under or over evaluated in in vitro conditions.