We have previously shown whole body autoregulation during normoxia in conscious areflexic rats in response to an acute increase and decrease in blood volume. In this study we used this technique to determine the effect of hypoxia and hyperoxia on whole body autoregulation. Rats with an arterial catheter for blood pressure measurement and an electromagnetic flow probe for cardiac output measurement were placed in a chamber with a controlled oxygen environment. Neurohumoral blockade was achieved with chlorisondamine (10 mg/kg), methscopolamine (0.5 mg/kg), captopril (1 mg/kg), and [d(CH2)5Tyr(Me)]arginine vasopressin (10 micrograms/kg). Hemodynamic variables were restored to normal with a constant norepinephrine infusion. Group 1 (n = 7) underwent a 6-min infusion of donor blood (0.9 ml) during hypoxia (Po2 = 52 +/- 3 mmHg) and hyperoxia (Po2 = 296 +/- 12 mmHg). Group 2 (n = 8) was subjected to a 6-min withdrawal of blood (0.9 ml) during hypoxia (72 +/- 2 mmHg) and hyperoxia Po2 = 258 +/- 8 mmHg). The slope of the pressure-flow relationship was used as an index of autoregulation so that a slope of 0 indicated complete autoregulation and a slope of 1 represented no autoregulation. The pressure-flow slopes with volume expansion were 0.54 during hypoxia and 0.15 during hyperoxia, while the slopes with volume contraction were 0.29 during hypoxia and 0.54 during hyperoxia. Thus, when arterial pressure was raised above normal, the autoregulatory capacity was greater during hyperoxia than with hypoxia. Conversely, when arterial pressure was lowered below normal, the autoregulatory capacity was greater during hypoxia than with hyperoxia.(ABSTRACT TRUNCATED AT 250 WORDS)