Adrenal response to tolbutamideinduced hypoglycemia was studied in 23 normal men born and living at high altitude (Morococha, Peru, 4,540 m) and compared to 23 men born and living near sea level. Fasting blood sugar was significantly lower in the high altitude dwellers (HAD) and this difference persisted throughout the test period. The blood glucose decrement at 30 min was significantly less in the HAD. Plasma cortisol response was significantly higher at 30 and 60 min in the HAD. Peak cortisol level occurred earlier in the HAD than in men living at sea level. The absolute cortisol increment at 30 min was significantly greater in the HAD. © 1975 by The Endocrine Society.