The rat working heart-brain stem preparation (WHBP) is an in situ preparation having many of the advantages associated with in vitro preparations while retaining cardiovascular response functionality and an eupnoeic respiratory motor pattern. The preparation is perfused arterially with an aqueous medium having a much lower oxygen-carrying capacity than blood. To evaluate the efficacy of the artificial perfusion in providing adequate gas exchange within the brain stem, we used polarographic PO(2) and pH microelectrodes to determine the tissue PO(2) and pH of the medulla oblongata at various depths. When the perfusate was equilibrated with 5% CO(2) and 95% O(2), average tissue PO(2) was 294 Torr and no hypoxic areas were encountered. Tissue pH was remarkably uniform throughout the tissue, and on average was only 0.04 +/- 0.02 pH units more acidic than that of the perfusate. Increasing the PCO(2) of the perfusate increased tissue PO(2) and decreased arterial resistance. Decreasing perfusate PCO(2) (while keeping pH constant) decreased tissue PO(2) and reduced the respiratory activity. These results suggest that arterial PCO(2), independent of arterial pH, is an essential variable in determining both respiratory drive and cerebrovascular perfusion. We conclude that the medulla of the WHBP is oxygenated and within a physiological pH, which accounts for the eupneic pattern of respiratory motor activity it generates. Furthermore, this preparation may be a useful model for exploring mechanisms of central chemoreception as well as the dynamics of the cerebral vasculature responses following changes in blood gases.