This study is the first step in characterizing ion uptake mechanisms of mosquito larvae from the Amazon region of Brazil. Hemolymph NaCl levels and rates of unidirectional Na(+) and Cl(-) uptake were measured in larvae of Aedes aegypti and Culex quinquefasciatus in a series of environmental manipulations that are known to challenge ion regulation in other aquatic animals. Despite being reared for numerous generations in dilute media (20 micromol L(-1) NaCl), both species were able to maintain high hemolymph NaCl concentrations, a departure from previous studies. Exposure to distilled water or high-NaCl media did not affect hemolymph ion levels, but pH 3 caused significant decreases in hemolymph Na(+) and Cl(-) levels in both species. Exposure to water from Rio Negro (pH 5.5), an organically rich but ion-poor body of water, did not disturb hemolymph Na(+) and Cl(-) levels or the uptake of these ions. Acute exposure to control media or Rio Negro water titrated to pH 3.5 caused inhibition of Na(+) uptake and stimulation of Cl(-) uptake in C. quinquefasciatus, but A. aegypti larvae experienced only a significant reduction of Na(+) uptake in Rio Negro/pH 3.5 treatment. The stimulation of Cl(-) uptake at low pH has been documented only in aquatic insects and differs from all other invertebrate and vertebrate species. A similar pattern of Na(+) uptake inhibition and Cl(-) uptake stimulation was observed in A. aegypti larvae exposed to bafilomycin A(1), a blocker of V-type H(+) ATPase. Culex quinquefasciatus larvae were unaffected by this drug. Both Na(+) and Cl(-) uptake were reduced when C. quinquefasciatus larvae were exposed to acetazolamide, indicating that H(+) and HCO(3)(-), derived from hydration of CO(2), are involved with Na(+) and Cl(-) uptake. Kinetic analysis of Na(+) and Cl(-) uptake in C. quinquefasciatus, A. aegypti, and Anopheles nuneztovari larvae indicate that these Amazonian species share similar high-capacity and high-affinity mechanisms. Comparison of the Amazonian C. quinquefasciatus with a Californian population provided evidence of both phenotypic plasticity and population disparity in Na(+) and Cl(-) uptake, respectively. When the California population of C. quinquefasciatus was reared in a medium similar to that of the Amazonian group (60 micromol L(-1) NaCl) instead of 4,000 micromol L(-1) NaCl, larvae increased both Na(+) uptake capacity (J(max)) and affinity (i.e., reduced K(m)), yet Cl(-) uptake did not change from its nonsaturating, low-capacity pattern. In the reverse experiment, Amazonian C. quinquefasciatus demonstrated plasticity in both Na(+) and Cl(-) uptake by significantly reducing rates when held in 4,000 micromol L(-1) NaCl for 3 d.