Experiments were performed to compare the diuretic and natriuretic response to a moderate saline load between 12-week-old conscious spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) male rats. The animals were infused intravenously with 0.9% NaCl, first for a 1-h equilibration period, at a rate of 20 microliters/min, then for a 2-h salt-loading phase at 40 microliters/min, and finally during a 1-h recovery at 20 microliters/min. Cumulative renal excretion of water and sodium was similar in SHR and WKY rats. In contrast, urinary kallikrein excretion (amidolytic assay) during the saline-expansion and recovery phases was significantly higher in SHR than in WKY rats. At the end of the salt-loading phase, plasma levels of atrial natriuretic peptide (ANP) were higher in SHR than in WKY rats (p less than 0.05). This was also true in animals infused with 5% dextrose instead of 0.9% NaCl. Splanchnic nerve activity remained stable throughout the study in SHR as well as in WKY rats. These data therefore indicate that SHR excrete water and sodium similarly to WKY rats when they are subjected to a moderate sodium load. Neither ANP nor the sympathetic nervous system seems to be directly implicated in the renal response to the moderate saline expansion. Whether the enhanced urinary kallikrein excretion observed in SHR reflects an active participation of the renal kallikrein-kinin system in their sodium handling remains uncertain.