The effect of O(2) partial pressure on the germination and the respiration of 12 cultivated species was studied. The reciprocal of the time necessary to observe rootlet emergence in 50% of the seeds was used to approach the germination rate. The maximum germination and respiration rates were reached in most seeds at O(2) pressures close to that of air. Decreasing the O(2) pressure produced a gradual decrease of the germination rate. The seeds could be classed in two groups according to their response to low O(2) pressures. Group I includes lettuce, sunflower, radish, turnip, cabbage, flax, and soybean: at O(2) pressures close to 2 kilopascals, the germination in this group was stopped and the adenylate energy charge was lower than 0.6. Group II includes rice, wheat, maize, sorghum, and pea. The germination rate of these seeds was also gradually decreased by lowering the O(2) partial pressure but germination still occured, very slowly, at 0.1 kilopascal; the adenylate energy charge remained higher than 0.6. These differences in the germination rates and adenylate energy charge values could not be explained by differences in the sensitivity of respiration to O(2).