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We have developed chronic toxicity reproductive 5-day tests to study the effects of Pb, as Pb(NO3)2, and Hg, as HgCI2, exposure in the freshwater rotifer Lecane quadridentata. We used ‘r’ (the instantaneous growth rate) as the endpoint. The test was performed using EPA medium at two food levels (105 and 106 cells/mL of Nannochioris oculata) at a controlled temperature of 25°C and an L/D cycle of 16:8 h. We monitored the levels of both metals, using atomic absorption, at days 0, 2, and 5 of each experiment. Our results showed that rotifers fed at higher food concentrations were capable of withstanding higher levels of metal toxicity than those fed at lower food concentrations (EC5O = 0.704 versus 0.664 mg/L-1 for lead, P < 0.05; EC5O = 0.057 versus 0.054 for mercury, P < 0.05). Our atomic absorption analysis showed that although 66% of lead nitrate can be taken up by algal cells efficiently and removed from the medium, rotifers also play an important role removing additional lead from the medium (up to 44% in some treatments). In the case of mercuric chloride, most of the mercury is bound by the salts contained in EPA medium or discarded by the organisms, and the remainder is removed by N. oculata in <48 h. [ABSTRACT FROM AUTHOR]