ABSTRACT. Grapefruits are highly susceptible to developing chilling injury (CI) symptoms during postharvest storage at low temperatures. The role of ethylene in the response of citrus fruits to low temperature is still controversial and whether the hormone is a defense response to cope with the cold-induced damage is not fully understood. Moreover, little is known about the ethylene signaling cascade in coldsensitive citrus fruit under chilling conditions. Therefore, the aim of this work was to study the transcriptional regulation of ethylene biosynthesis, perception and response during storage at 2°C of the yellow ?Marsh? and the red ?Star Ruby? grapefruits. To better understand the regulation of cold- or chilling-induced ethylene production, the effect of the ethylene action inhibitor, 1-MCP, on the expression of ethylene biosynthetic and perception genes was also analyzed. Cold storage markedly stimulated the expression of ethylene biosynthetic genes; ACC synthase 1 and 2 (ACS1 and ACS2) and ACC oxidase (ACO) in the peel of both cultivars. Expression of ACS1, ACS2 and ACO were overstimulated in the peel of 1-MCP-treated fruit, indicating a negative feedback control of ethylene on the process. Expression of three members of the ETR gene family was also affected by low temperature, but expression of ETR3 gene experienced the major induction by cold stress in the peel of fruit of both cultivars. Pre-treatment with 1-MCP did not show a clear and consistent effect on the expression of the ETR genes. These results illustrate the complex regulation of the different members of ethylene biosynthetic, signaling and response pathway by cold stress in the peel of grapefruits.