Silver nanoparticles (AgNPs) are the most commonly used metallic nanoparticles in industrial applications, including medical and consumer products. In the recent years, however, concerns regarding their environmental and health impacts have emerged. Aquatic organisms are of special concern since water bodies often serve as sinks for anthropogenic activities. This study assessed the effects of AgNPs on the activities of the gill Na(+)/K(+)-ATPase and erythrocyte acetylcholinestrase (AChE), as well as the plasma biochemistry in adult zebrafish (Danio rerio). In an acute exposure scenario the fish were exposed for 4d to 16.76 mg/L AgNPs, which was the 96 h LC50 value determined in preliminary experiments. In a prolonged exposure scenario the fish were exposed for 1, 2, or 3 weeks to AgNPs at concentrations of 2 and 4 mg/L, corresponding to the 1/10th and 2/10th of the 96 h LC50 value. Generally the activity of the gill Na(+)/K(+)-ATPase decreased, but this was only significant starting at 14 d of the prolonged exposure scenario, whereas the activity of the erythrocyte AChE was significantly decreased in both exposure scenarios. Finally, the plasma electrolytes levels were reduced and the plasma glucose and cortisol levels were increased in exposed fish. This study demonstrates that AgNPs could inhibit the activities of Na(+)/K(+)-ATPase and AChE, thus interfering with the proper ionoregulation and neuroregulation, respectively, and act as stressors.