The rise in urinary pCO2 above blood pCO2 which occurs in response to bicarbonate loading (i.e. the urine to blood (U-B) pCO2 gradient), is used with increasing frequency as an index of collecting duct hydrogen ion secretion. We recently proposed, however, that the U-B pCO2 gradient is not an appropriate index of collecting duct hydrogen ion secretion when blood pCO2 is altered acutely. This issue was further investigated by examining the effect of chronic hypercapnia on urinary pCO2 generation. In rats exposed to chronic hypercapnia induced by breathing 10% CO2 for 3 days in an environmental chamber, acute sodium bicarbonate infusion resulted in a U-B pCO2 lower than that of normocapnic control rats (11 +/- 4.6 and 30 +/- 1.8 mm Hg, p less than 0.001). This finding could be interpreted to indicate that collecting duct hydrogen ion secretion is depressed in rats with chronic hypercapnia. The urinary pCO2 of rats with chronic hypercapnia was lower than that of the blood (54 +/- 6.0 and 86 +/- 1.2 mm Hg, p less than 0.005, respectively). In these rats, NaHCO3 infusion, while blood pCO2 was kept constant, elicited a marked rise in urine pCO2 (from 54 +/- 6.0 to 104 +/- 6.0 mm Hg, p less than 0.005) which was not significantly different from that observed in normocapnic control rats. The infusion of carbonic anhydrase resulted in a comparable fall in urine pCO2 in hypercapnic and normocapnic rats (-27 +/- 5 and -30 +/- 3 mm Hg).(ABSTRACT TRUNCATED AT 250 WORDS)