The isolated transverse brainstem slice preparation of neonatal mice is employed to investigate the function of theophylline, a competitive nonselective phophodiesterase inhibitor and adenosine receptor antagonist, on the hypoxic ventilatory response. Brainstem slices are isolated from neonatal mice (4-8 days old) and superfused with artificial cerebrospinal fluid (aCSF), equilibrated with a hyperoxic gas mixture (95% O₂, 5% CO₂) as a control, and anoxic mixture (0% O₂, 5% CO₂, 95% N₂) to create severe hypoxia at the tissue level. Using suction electrodes, extracellular population activities of respiratory neurons is recorded from brainstem slices in the region of the pre-Bötzinger Complex (preBötC), a site of inspiratory rhythm generation. One goal of this study is to detect if the theophylline increases in vitro respiratory activity under control oxygen conditions. Another goal is to determine the extent to which theophylline reverses the respiratory depression during severe hypoxia.