Oral intake of NH₄Cl has been used to study chronic metabolic acidosis in mammals. This, however, cannot be achieved in the herbivore rabbit on alkali-rich standard food (Bungart et al., Atemw.-Lungenkrkh. 21, 365, 1995). To find pre-conditions for chronic metabolic acidosis in this species, two groups (each N=9) of rabbits were fed low-alkali diet (PCC) with and without additional daily oral NH₄Cl ingestion of 4.2 ±0.4 mEq·kg^-1 (mean ±SEM) and were compared to controls on normal food. Fractional renal base re-absorption was significantly (P<0.001) enhanced in both diet groups from 86.8 ±1.2% under control con-ditions to 98.3 ±0.7% and 98.8 ±0.7%, respectively. Feeding PCC alone did not significantly change the systemic base excess from 0.8 ±0.6 (control) to 1.0 ±0.8 mM, but PCC plus NH₄Cl was followed by a distinct reduction to -6.4 ±0.9 mM (P<0.001). These data confirm that systemic acid-base balance in the rabbit is not disturbed by a base-deficient diet alone (Kiwull-Schöne et al., Eur. J. Nutr. 44, 499, 2005), but maximum stress on the renal base saving mechanism is prerequisite to obtain manifest systemic acidosis during additional NH₄Cl load.