Natriuretic Peptides (NPs) are evolutionarily conserved hormones that affect blood pressure and fluid volume through membrane bound guanylate cyclase (GC)-linked natriuretic peptide receptors-A and -B (NPR-A and NPR-B, respectively) in a variety of vascular, renal and other tissues. The principal physiological stimulus for cardiac NPs in fishes is somewhat debated between two prominent theories; regulation of salt balance (osmoregulatory hypothesis) or prevention of volume expansion (cardioprotective hypothesis). In the present study we examined atrial and ventricular expression of trout NPs, atrial (ANP), brain (BNP) and ventricular (VNP) using northern (mRNA), western (NP pro-hormone) and qPCR (GC-NPR-A and -B mRNA) analysis following independent manipulation of plasma salt and volume levels after chronic exposure to freshwater (FW; volume loaded, salt depleted), saltwater (SW; volume depleted, salt loaded), or freshwater trout fed a high salt diet (FW-HSD; volume and salt loaded). We also measured NP transcriptional response to acute (2 h) volume expansion with dialyzed plasma (VE; 80% blood vol) or volume depletion by hemorrhage (VD, 20% blood volume every 30 min for 2 h) with real-time PCR. In essentially all instances, increased expression of the NP system was associated FW-HSD or plasma expansion. There were no differences in NP expression between chronically adapted FW and SW fish and hemorrhage decreased atrial ANP and VNP mRNA. These results indicate that rainbow trout cardiac NPs and cardiovascular GC-NPRs respond principally to volume, not salt overload and this suggests that the primary function of trout cardiac NP system is to protect the heart.