Two experiments were conducted to investigate the response of two populations of chickens, divergently selected for high and low oxygen consumption, to varying protein levels. Protein levels used for Experiments 1 and 2 were 16, 18, 20, and 22% and 20, 22, 24, and 26%, respectively. The birds were weighed initially and weekly to 4 weeks of age. Feed consumption was ad libitum with amounts consumed recorded weekly. Gross feed efficiency was computed per pen as the difference between initial and weight at 4 weeks divided by feed consumed adjusted for spillage. The results from these experiments indicate that at low protein levels both lines were equally efficient, but, as the level of protein was increased, the low line was increasingly more efficient while the high line became less efficient. Gain for the low O2 birds was greater than the high O2 birds in all cases with both lines increasing in gains equally with increased protein levels.