The present study was aimed at investigating the atrial natriuretic peptide (ANP) and urinary responses to acute perturbations in fluid balance and the vascular function in diabetes mellitus (DM). DM was induced in rats by treatment with streptozotocin (50 mg/kg, i.p.). Ten weeks later, the plasma ANP concentration measured in the conscious state was significantly higher in DM group (27.5 ± 3.9 pg/mL) than in the control (15.4 ± 2.6 pg/mL), while the atrial tissue contents of ANP were lower. In response to acute extracellular volume expansion (VE), amounting up to 5% of body weight over 45 min, under thiopental anesthesia (50 mg/kg, i.p.), the magnitude of increase in plasma ANP was lower in the DM group than in the control (56.8 ± 25.2 vs. 189.1 ± 53.6% increases over the basal). Urinary sodium excretion during VE was also lower in the DM group. Acetylcholine-induced relaxation of the isolated aortic rings was attenuated in the DM group, which was partially restored by L-arginine-supplementation (2 g/L in drinking water). These results suggest that body fluid homeostasis and vascular functions are unfavorably altered in DM.