A retrospective survey performed in 100 men born at sea level, residing at 12,398 feet of altitude for 2 to 15 years, has provided a basis for studying the systemic blood pressure changes possibly associated with prolonged residence in a hypoxic environment. Comparison of the blood pressure at the initial and final examinations revealed: (1) decrements of 10 mm. Hg or more for systolic and diastolic pressures in the whole sample in 56 and 46 per cent of the subjects, respectively; (2) significant differences except for diastolic pressure in the subjects with the longest period of residence at high altitude; and (3) in lowlanders, a response of blood pressure to aging at this altitude differing from that at sea level. In general, the final blood pressure closely resembled that observed in healthy natives of the same altitude. Since diet, physical activity on the job, and habits in these subjects were similar to those of their original countries and quite different from those of the Andean population, it seems probable that these findings are causally related to an environmental hypoxic stimulus. Functional or anatomic vascular changes decreasing peripheral vascular resistance to blood flow would be the principal determinant of the observed differences. © 1969.