Embolized nitrogen appears in alveolar gas during clinical and experimental venous air embolism (VAE). Since early detection of VAE is believed to reduce morbidity and mortality, this study was done to compare the sensitivity of end-tidal nitrogen (ETn2) monitoring with other detection methods in current clinical use—precordial Doppler (PD), end-tidal CO2(ETCO2), and pulmonary artery pressure (PAP). Ten mongrel dogs (10–17 kg) were anesthetized, placed in the supine position, immobilized, and ventilated (FIO21.0; PaCO235–40 mmHg). Anesthesia and muscle relaxation were maintained with constant infusions of thiamylal and pancuronium. Maintenance fluids were administered at 5 ml · kg−1· h−1, Mean arterial pressure (MAP), PAP, and ETN2and ETCO2(Medspec II® mass spectrometer) were displayed on a strip chart recorder. The dogs were divided into two equal groups and given either a step-wise sequence of 1-min air infusions (0.1–1.5 ml · kg−1· min−1) or 5-s bolus air injections (0.25–1.0 ml · kg−1). Changes in PD sounds occurred in all animals at all air doses. Changes in cardiovascular variables and PaO2were minimal. The threshold dose for ETCO2and ETN2to reach significance was 0.1 and 0.25 ml · kg−1, respectively, while PAP increases were significant at >0.5 ml · kg−1air doses. The time to maximum change (Δmax) ETN2was 30–90 s earlier than Δmax ETCO2(P< 0.05) and 6–105 s earlier than Δmax PAP. The Δmax for all variables was dose related and statistically significant except for the smallest infusion VAE, where only ETCO2was significantly changed. At 0.5 ml · kg−1and greater air doses, increases in ETN2after bolus VAE were double those following infusion of the same amount of air. The magnitude of the changes in ETCO2and PAP were comparable following bolus and infusion VAE. It is concluded that changes in ETN2following low-dose infusion VAE are less sensitive than changes in ETCO2, while during the bolus doses studied, they are equally sensitive. Changes in ETN2and ETCO2are more sensitive than changes in PAP during bolus and infusion VAE. Continuous mass spectrometry monitoring of ETN2may not provide early warning of air entry after small emboli, however, when a significant increase in ETN2occurs following VAE, it precedes changes in ETCO2and PAP.