Duplicate determinations of arterial oxygen and carbon dioxide tensions were made in seventy adult patients of both sexes awaiting elective surgery. No difference in arterial oxygen tension was noted between the sexes but a marked inverse correlation between age and arterial oxygen tension was demonstrated. When this relationship was evaluated from the duplicate estimations the resultant regression equation was such that Pao2 = 102.5–0.22 (age). No statistically significant differences were noted between the Paco2 values of patients of different sexes and age groups.