Utilizing a well-established porcine model of cardiac arrest, we found that end-tidal CO2 concentration (ETco2) strikingly decreased to approximately 24% of control levels, immediately after cardiac arrest and before precordial compression. During precordial compression, ETco2 progressively increased to 46% of control values in successfully resuscitated animals but only to 26% in animals which failed to respond to resuscitation efforts. After successful resuscitation, ETco2 rapidly returned to baseline values. These data indicate that ETco2 may be a useful monitor for assessing the adequacy of CPR.