Carbon dioxide responsiveness was determined in 126 normal, young adults. The mean +/- SE slope of the line relating PCO2 to minute volume of ventilation was 2.60 +/- 0.107 liter per min-mm Hg. Although a wide range of responses was noted, 80 percent of the subjects had a value for the slope between 1.5 and 5.0 liter per min - mm Hg, with 16.6 per cent having values lying below and 4 per cent above this range. Women were significantly less responsive than men. Low, but significant, correlations were found between the slope and vital capacity, and between the slope and maximal breathing capacity, but not between the slope and height, weight, age, or smoking habits. Additional findings included an increase in the slope with prolonged rebreathing, possibly due to a peripheral chemoreceptor effect; absence of metabolic acidosis during 4 min of rebreathing; excellent short-term reproducibility of results in 111 subjects; and good longer-term reproducibility in most of 10 subjects retested 9 to 24 months later.