The positron-emitting isotope 11C was used to label CO2 for studies of metabolic fixation and equilibration after a single-breath inhalation by rats. Metabolic fixation and loss of the label via exhalation caused the metabolized fraction of the label in the brain to rise to 30.1 ± 0.7% within 30 minutes. The T12 for equilibration of the label between blood and brain was 1.95 minutes. When the label was 95% equilibrated, 12% was metabolically trapped by brain, and when only 5% was trapped, the blood-brain equilibration process was only 50% complete. Labeled CO2 thus has limited usefulness as an acid-base or metabolic tracer for positron-emission tomography.