The toxicity of manufactured silver nanoparticles (AgNPs) has been widely studied, but the influence of AgNPs on the major ions (such as sodium [Na] and calcium [Ca]) regulations are unknown. In the present study, a freshwater cladoceran Daphnia magna was exposed to commercial AgNPs coated with polyvinylpyrrolidone. After 48 h, the Na body content was significantly reduced by AgNO3 exposure, but the Ca body content was significantly increased under AgNO3 and AgNP exposures, respectively. No effect was observed on the body concentrations of Na and Ca at 50 to 500 µg/L AgNPs with 1-µM cysteine addition. Exposure of AgNO3 and AgNPs inhibited the Na influx and elevated the Na efflux. In contrast, their exposure increased the Ca influx, but did not affect the Ca efflux. The results of the present study demonstrated the significant influences of AgNO3 and AgNPs (without cysteine) on Na and Ca regulations. Such effect of AgNPs on Na and Ca regulation disappeared after cysteine addition, indicating that the soluble Ag released from AgNPs played a major role in the ionoregulatory dysfunction.