There have arisen two schools of thought concerning the role of ethylene in fruit maturation: the classic view of Kidd and West (26) and Hansen (22) that ethylene is a ripening hormone, and a recent interpretation by Biale et al. (7, 3, 4) that it is a by-product of the ripening process. The original presentation of the by-product theory in this journal (7) was tempered with the reminder that 0.1 ppm ethylene may stimulate ripening, so that "in the absence of any information correlating the internal ethylene content with the rate of ethylene production, one can advance the argument that small quantities sufficient to induce ripening are produced prior to the rise of respiration, but measurable amounts are detected only after the onset of the climacteric." The development of highly sensitive gas chromatographic instruments makes it feasible to appraise critically those instances in which fruits have been reported to produce ethylene not at all or only after the climacteric has started, and also to determine the content of ethylene within a fruit at the onset of the rise in respiration. Results of such experiments are reported in this communication, and they have a direct bearing on the problem of whether or not ethylene is a natural ripening hormone.