The Canadian Rare Earth Element (REE) Network aims producing and processing 20% of the global supply of critical REEs in Canada by 2018 which may lead to increased REE emissions into the environment. The potential hazard of elevated concentrations of REEs to biota is still insufficiently investigated and is fraught with potential methodological problems. In the current presentation, three aspects of REEs toxicity to Daphnia magna will be discussed: 1) effect of test medium on acute toxicity of REE; 2) chronic toxicity of REE; 3) toxicity and bioaccumulation of REE rich rock leachates. Daphnia magna is a particle-feeding zooplankton species with high potential of accumulating REEs. Acute and chronic toxicity of La, Ce, Pr, Nd and Gd nitrates to D. magna was evaluated according to OECD202 and OECD 211 guidelines. Acute toxicity was tested in OECD202 artificial freshwater (AFW) and natural lake water and chronic toxicity in natural lake water. To evaluate sedimentation, REEs' content in supernatants was measured by total reflection X-ray fluorescence spectroscopy (TXRF). Acute toxicity (48 h EC50 in mg REE/L) to D. magna, calculated based on nominal metal concentrations, was 19-31 in AFW and >50 in lake water. 48 h EC50 (mg REE/L), based on measured concentrations, was 0.2-1.5 in AFW and >0.2 in lake water. Chronic toxicity to D. magna , evaluated in lake water, was 0.3-0.5 mg REE/L. TXRF analyses showed that sedimentation of REEs was higher in AFW (at low concentrations) and at high test concentrations (in both test media). Bioavailability of REE from rock leachates was measured from REE rich monazite (MON) or allanite (ALA) bearing pegmatite rock material from Quebec (Canada). The materials were leached with artificial rainwater according to ISO/TS 21268-2 guideline. New leachate was produced at 6 leaching cycles performed during 28 days and analysed with ICP-MS and ICP-OES. Filtered eluates from day 1 and 28 were tested for acute toxicity (OECD202) to D. magna and those from day 14 to measure REE bioaccumulation in D. magna. Toxicity was seen only for MON eluates with significantly lower REE concentration compared to ALA. The toxicity did not decrease with weathering despite the decreasing REE content in the leachates. Thus, the toxicity of MON rock eluent was probably not (solely) induced by REE content. REE concentration in the eluates was low (<0.2 mg/L) compared to the concentration in the rocks (7000−12000 mg/kg). Almost no accumulation of REE from rock eluates was seen. In conclusion, in OECD202 and 211 test settings, REE solutions were more stable at low concentrations in organics-containing natural water (environmentally relevant conditions) compared to mineral artificial OECD202 test medium. The release of REEs from rock material in the leaching tests was low and decreased in time. Toxicity of rock eluents to Daphnia magna was not (only) related to REE content but (also) to additional factors that are yet to be specified. This work was supported by Estonian Research Council grant IUT23-5. The authors would like to acknowledge support by the COST Action TD1407 by means of a STSM Grant to Marge Muna.