The biokinetics of Cu in Daphnia magna, including dissolved uptake, assimilation and efflux, has been determined using a gamma 67^Cu radiotracer methodology. However, this gamma emitting radioisotope is not readily available due to its very short half-life. In the present study, we employed a stable isotope tracer （65^Cu） to determine the Cu biokinetics and compared our results to those determined using 67^Cu. The dissolved uptake rate constant of 65^Cu was 3.36 L/（g.day）, which is higher than that of 67^Cu （1.32 L/（g.day））. With increasing food concentrations from 2×10^4 to 1×10^5 cells/mL, the Cu assimilation efficiency （AE） decreased from 46% to 11%, compared to a decrease from 27% to 16% when determined using 67^Cu. The effiux of Cu from Daphnia magna was quantified following both dissolved and dietary uptake. The efflux of waterborne Cu was comparable to that of dietborne Cu and the effiux rate constant （0.32-0.52 day^-l） was higher than that determined by 67^Cu （0.19-0.20 day^-1）. By considering different water properties and handling procedure between the two experiments, we believe that these differences are reasonable. Overall, this study demonstrated that the enriched stable isotope tracer technique is a powerful tool to investigate metal bioavailability and maybe a good alternative to radioactive measurements.
