1. A quantitative study of ethylene production in relation to the respiration of pears has been made. The emanation of ethylene in air at 20⚬ C., as well as at higher and lower temperatures in modified atmospheres, has been studied. 2. In fruit in air at 20⚬ C. the rate of ethylene production increases during the climacteric rise in respiration, reaches a peak at the respiratory climax, then declines during the post-climacteric period. During the climacteric rise, ethylene production increases seven- to eighty-fold, while rate of respiration approximately doubles. 3. Each variety was found to have a characteristic maximum rate of production. The maximum rate for Bartlett, a variety which maintains its capacity to ripen for only a short period of time when kept at a storage temperature of 0⚬ C., is 3.25-4.48 ml. per kg.-day. The maximum rate for Anjou, a variety which maintains its capacity to ripen for a long period of time when kept at cold storage temperatures, is 0.57-0.78 ml. per kg.-day. The maximum ...