Silver (Ag) nanoparticles are used as antimicrobial adjuvant in various products such as clothes and medical devices where the release of nano-Ag could contaminate the environment and harm wildlife. The purpose of this study was to examine the sublethal effects of nano-Ag and dissolved Ag on Oncorhynchus mykiss rainbow trout. Hepatic Ag contents and changes in gene expression were monitored to provide insights on bioavailability and mode of action of both forms of silver. Fish were exposed to increasing concentrations (0.06, 0.6 and 6 µg L(-1)) of nano-Ag (20 nm) and silver nitrate (AgNO(3)) for 96 h at 15°C. A gene expression analysis was performed in the liver using a DNA microarray of 207 stress-related genes followed by a quantitative polymerase chain reaction on a selection of genes for validation. The biochemical markers consisted of the determination of labile zinc, metallothioneins, DNA strand breaks, lipid peroxidation (LPO) and vitellogenin-like proteins. The analysis of total Ag in the aquarium water revealed that nano-Ag was mostly aggregated, with 1% of the total Ag being dissolved. Nevertheless, hepatic Ag content was significantly increased in exposed fish. Indeed, dissolved Ag was significantly more bioavailable than nano-Ag only at the highest concentration with 38 ± 10 and 11 ± 3 ng Ag mg(-1) proteins for dissolved and nano-Ag respectively. Exposure to both forms of Ag led to significant changes in gene expression for 13% of tested gene targets. About 12% of genes responded specifically to nano-Ag, while 10% of total gene targets responded specifically to dissolved Ag. The levels of vitellogenin-like proteins and DNA strand breaks were significantly reduced by both forms of Ag, but DNA break levels were lower with nano-Ag and could not be explained by the presence of ionic Ag. Labile zinc and the oxidized fraction of metallothioneins were increased by both forms of Ag, but LPO was significantly induced by nano-Ag only. A discriminant function analysis revealed that the responses obtained by biochemical markers and a selection of ten target genes were able to discriminate completely (100%) the effects of both forms of Ag. Exposure to nano-Ag involved genes in inflammation and dissolved Ag involved oxidative stress and protein stability. Hence, the toxicity of Ag will differ depending on the presence of Ag nanoparticles and aggregates.