Transcutaneous oxygen tension (Ptco2) and transcutaneous carbon dioxide tension (Ftcco2) were monitored in 60 patients undergoing neurosurgical procedures. Twenty-six patients were in the sitting position and underwent routine monitoring for air embolism. Seventeen episodes of air embolism were diagnosed by precordial Doppler ultrasound or transesophageal echocardiography, and the Ptco2decreased early during the course of each episode. The mean Ptco2decrease was 48 ± 35 mmHg. During ten episodes the end-tidal carbon dioxide tension (PETco2) decreased but only after the Ptco2had already begun to decrease. Ptcco2increased during air embolism but PETco2changes preceded the change in PtCco2by 1–2 min. Transcutaneous values during air embolism were verified with simultaneous arterial blood gas values during six air embolism episodes. A strong positive correlation was found between transcutaneous and arterial oxygen and carbon dioxide tensions. Correcting the Ptcco2by the patient's baseline Ptcco2/Paco2ratio, Ptcco2monitoring correctly reflected hypocarbia, normocarbia, and hypercarbia in 92% of the-cases. Ptco2monitoring was useful in detecting venous air embolism and may respond sooner than PETco2. Ptcco2monitoring was not useful as an early detector of air embolism.