The contribution of the renin–angiotensin system (RAS) to Na+ uptake was investigated in larval zebrafish (Danio rerio). At 4 days post fertilization (dpf), the level of whole-body angiotensin-II (ANG-II) was significantly increased after 1- or 3-h exposure to acidic (pH=4.0) or ion-poor water (20-fold dilution of Ottawa tapwater), suggesting rapid activation of the RAS. Long-term (24 h) treatment of 3 dpf larvae with ANG-I or ANG-II significantly increased Na+ uptake which was accompanied by an increase in mRNA expression of the Na+-Cl− cotransporter (zslc12a10.2). Induction of Na+ uptake by exposure to ANG-I was blocked by simultaneously treating larvae with lisinopril (an angiotensin-converting enzyme inhibitor). Acute (2 h) exposure to acidic water or ion-poor water led to significant increase in Na+ uptake which was partially blocked by the ANG-II receptor antagonist, telmisartan. Consistent with these data, translational knockdown of renin prevented the stimulation of Na+ uptake following exposure to acidic or ion-poor water. The lack of any effects of pharmacological inhibition (using RU486), or knockdown of glucocorticoid receptors on the stimulation of Na+ uptake during acute exposure to acidic or ion-poor environments, indicates that the acute effects of RAS occur independently of cortisol signaling. The results of this study demonstrate that the RAS is involved in Na+ homeostasis in larval zebrafish.