Normal pregnancy is characterized by sodium and water conservation and an increase in plasma volume, yet the natriuretic and diuretic responses to acute saline volume expansion (VE) are intact in pregnant rats. Nitric oxide (NO) has been suggested to play a role in renal and cardiovascular adaptations to normal pregnancy. Therefore, the objective of this study was to determine the role of NO in the natriuretic and diuretic responses to VE during pregnancy. Infusion of N^G-monomethyl-L-arginine (L-NMMA; 15 mg/kg bolus followed by 500 µg/kg/min continuous infusion) was utilized to inhibit NO synthesis. Nine groups of Sprague-Dawley (SD) rats were studied. Nonpregnant (NP-VE, n=7), midterm pregnant (MP-VE, n=8), and late-term pregnant (LP-VE, n=7) SD groups that underwent VE alone after a control period; NP-L-NMMA (n=7), MP-L-NMMA (n=8), and LP-L-NMMA (n=7) SD groups that were infused with L-NMMA after a control period; and another three groups of SD rats (NP-VE-L-NMMA, n=8; MP-VE-L-NMMA, n=7; and LP-VE-L-NMMA, n=12) that underwent simultaneous VE and L-NMMA infusion (VE-L-NMMA) after a control period. FE_Na was 7.22±1.03% for NP-VE, 9.89±1.85% for NP-L-NMMA, and 17.66±1.85% for NP-VE-L-NMMA (P<0.05 vs. NP-VE and NP-L-NMMA); 6.61±1.07% for MP-VE, 7.99±1.92% for MP-L-NMMA, and 10.24±1.91% for MP-VE-L-NMMA (NS vs. MP-VE and MP-L-NMMA); 8.20±1.92% for LP-VE, 8.09±0.70% for LP-L-NMMA, and 7.57±1.11% for LP-VE-L-NMMA (both NS vs. LP-VE and LP-L-NMMA). Renal interstitial hydrostatic pressure (RIHP) increased in all groups studied from control to experimental periods; however, the change in RIHP (RIHP) was significantly greater in all nonpregnant groups as compared to pregnant groups with similar experimental intervention (i.e. VE, L-NMMA, or VE- L-NMMA). In conclusion, the natriuretic and diuretic responses to volume expansion and L-NMMA infusion were additive in nonpregnant, but not in pregnant rats indicating a possible lower ability of pregnant rats to respond to combined significant natriuretic and diuretic stimuli.