An automated system for pO 2 analysis based upon phosphorescence quenching was tested. The system was calibrated in vitro with capillary samples of saline and blood. Results were compared to a conventional measuring procedure wherein pO 2 was calculated off-line by computer fitting of phosphorescence decay signals. PO 2 measurements obtained by the automated system were correlated (r 2 = 0.98) with readings simultaneously generated by a blood gas analyzer, accuracy being highest in the low (0-20 mm Hg) and medium pO 2 ranges (21-70 mm Hg). Measurements in in vivo studies in the hamster skin-fold preparation were similar to previously reported results. The automated system fits the phosphorescence decay data to a single exponential and allows repeated pO 2 measurements in rapid sequence.