The action of respiratory and metabolic acid-base disturbances on the O2-Hb affinity was studied in rabbits and cats. Blood samples of both species were exposed to in vitro pH-changes, which were either achieved by variation of PCO2 (2.8-8.3 kPa) at constant lactic acid concentration, or by addition of lactic acid (5-14 mmol X 1(-1) at constant PCO2. The PO2 at halfsaturation (P50) and the Hill's n were determined from O2-Hb dissociation curves (ODC) in a range between 20 and 80% SO2. Under standard conditions (T = 311 K, PCO2 = 5.33 kPa, pH = 7.4), the average P50 value was 4.66 +/- 0.05 kPa in rabbits, that is slightly higher than reported by others, and 5.17 +/- 0.03 kPa in cats. The average values of Hill's n were 2.91 +/- 0.02 and 2.95 +/- 0.03 for rabbits and cats, respectively. When plasma pH was varied by CO2, the resulting classical CO2 Bohr factor phi CO2 = delta lgP50/delta pH50 was distinctly higher in cats (-0.560 +/- 0.006, n = 25) than in rabbits (-0.504 +/- 0.014, n = 22), although in the latter species being even higher than reported elsewhere. Concomitant metabolic acidosis did not significantly affect phi CO2, but shifted the P50 at a given plasma pH to lower values. Substitution of lactic acid with equimolar amounts of sodium lactate left both phi CO2 and P50 unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)