Previously, bicarbonate transport was measured in isolated perfused rabbit cortical collecting ducts (CCD) and outer medullary collecting ducts (OMCD). Rabbit CCD either absorbed or secreted bicarbonate in vitro, depending on whether the animals were treated with NH4Cl or NaHCO3, but the OMCD absorbed bicarbonate regardless of the treatment. The general significance of these findings (particularly the bicarbonate secretion) was questioned because rabbits are herbivores that normally excrete alkaline urine. Therefore, we have now studied rats, an omnivorous species, that normally excrete acid urine. The overall pattern of bicarbonate transport in rats was similar to that previously found in rabbits. CCD from rats given NaHCO3 initially secreted bicarbonate, but those from rats given NH4Cl absorbed bicarbonate. Rat OMCD all absorbed bicarbonate, regardless of the treatment. The significant differences between the results with rats and rabbits were 1) a marked shift in bicarbonate transport in control and bicarbonate-loaded rat (but not rabbit) CCD with time of perfusion in vitro from secretion toward absorption; this implies an additional regulatory mechanism in rats; and 2) rat OMCDs absorbing bicarbonate more than three times faster than rabbit OMCD. These results provide additional evidence that conditioned changes in cortical collecting duct bicarbonate transport, now observed in two different species, play a significant role in the control of net acid excretion.