To test the role of blood flow in tissue hypoxia-related increased veno-arterial PCO(2) difference (DeltaPCO(2)), we decreased O(2) delivery (&Ddot;O(2)) by either decreasing flow [ischemic hypoxia (IH)] or arterial PO(2) [hypoxic hypoxia (HH)] in an in situ, vascularly isolated, innervated dog hindlimb perfused with a pump-membrane oxygenator system. Twelve anesthetized and ventilated dogs were studied, with systemic hemodynamics maintained within normal range. In the IH group (n = 6), hindlimb DO(2) was progressively lowered every 15 min by decreasing pump-controlled flow from 60 to 10 ml. kg(-1). min(-1), with arterial PO(2) constant at 100 Torr. In the HH group (n = 6), hindlimb DO(2) was progressively lowered every 15 min by decreasing PO(2) from 100 to 15 Torr, when flow was constant at 60 ml. kg(-1). min(-1). Limb DO(2), O(2) uptake (VO(2)), and DeltaPCO(2) were obtained every 15 min. Below the critical DO(2), VO(2) decreased, indicating dysoxia, and O(2) extraction ratio (VO(2)/DO(2)) rose continuously and similarly in both groups, reaching a maximal value of approximately 90%. DeltaPCO(2) significantly increased in IH but never differed from baseline in HH. We conclude that absence of increased DeltaPCO(2) does not preclude the presence of tissue dysoxia and that decreased flow is a major determinant in increased DeltaPCO(2).