Acute saline volume expansion (VE) in the developing animal is associated with a blunted natriuretic response when compared with that in adults. Recent studies have suggested that renal interstitial hydrostatic pressure (RIHP) plays an important role in mediating VE-induced natriuresis in the adult. The purpose of our study was to determine whether abnormalities in the RIHP response to VE could be involved in the blunted natriuretic response in the developing animal. The effect of an acute saline load (5% body wt) on RIHP and sodium excretion was examined in adult pigs (>70 d) and piglets (26-43 d). In response to an acute saline load, the piglets excreted significantly less sodium than the adults (1.7 ± 0.5versus3.8 ± 0.7 µmol/ min/g kidney wt). The increase in fractional excretion of sodium in response to VE was also significantly less in the piglets. There was no significant change in GFR in either group. Associated with the blunted natriuretic response in the piglet was an absence of an increase in RIHP (6.1 ± 1.1versus6.1 ± 1.6 mm Hg) in response to VE. In contrast, the adult pig, which exhibited a normal natriuretic response, showed a significant increase in RIHP (8.8 ± 1.3 to 12.3 ± 1.5 mm Hg) during VE. When RIHP was prevented from increasing during VE in the adult pigs, the natriuretic response was significantly attenuated, as it was in the piglets. The results of our study indicate that the piglet does not exhibit a normal adult RIHP response to VE. The absence of any discernible increase in RIHP during VE may explain, in part, why piglets have a limited ability to excrete a saline load.