The Gas STAT is a monitor which continuously measures pO₂, pCO₂ and pH inline during cardiopulmonary bypass. A clinical evaluation of the inline monitor compared to the standard method of monitoring, that is, intermittent isolated blood sampling and distant measurement on a blood gas analyser (BGA), was undertaken. During stable moderate hypothermia, 115 paired measurements were made in 54 patients undergoing coronary artery or heart valve surgery. The correlations between pO₂, pCO₂ and pH, as measured by the BGA and the inline monitor, were respectively 0.536 (P<0.001), 0.811 (P<0.001) and 0.826 (P<0-001). pCO₂ as measured by the inline monitor was significantly lower than the BGA ( P<0.001 ) whereas pH was significantly higher (P<0.001). There was no significant difference between pO₂ measurements. Possible explanations for the finding of less than complete correlations are discussed and it is suggested that although inline monitoring is of value in the control of PO₂, pCO₂ and pH during cardiopulmonary bypass, it should not be used without recourse to the standard method of blood gas analysis.