We studied the arterial blood gas determinations done on the first hospital day in 14 narcotic addicts with bacterial endocarditis (group 1) and six addicts with other medical complications of narcotic addiction (group 2). The arterial oxygen tension (PaO2) was 67.3 ± 3.3 mm Hg (SEM) in group 1, and 75.0 ± 6.4 mm Hg in group 2, with no statistically significant difference (P< .20) between the groups. The arterial CO2tension (PaCO2), 27.8 ± 1.7 mm Hg, in group 1 was significantly lower (P< .005) than that of group 2, 40.1 ± 3.7 mm Hg. The arterial blood pH, 7.47 ± 0.01 units, in group 1 was significantly higher (P< .025) than that of group 2, 7.36 ± 0.06 pH units. Abnormal blood gas values were found in each of the patients in group 1 with a normal admission chest roentgenogram. Arterial blood gas determinations may be useful in the initial examination of ill narcotic addicts.