During the past several years, substantial progress in understanding the receptors and signal transduction processes for peptidyl leukotrienes has been reported. Receptors have been identified and characterized, the major steps in the signal transduction pathway have been described, and the genetic and epigenetic regulatory processes have been characterized. Very recent studies have defined the mechanisms by which LTE4 acts as a partial agonist at the LTD4 receptor. The cloning of the genes for the proteins involved in the major steps of the signalling process has also been initiated. Stanley Crooke and co-authors summarize this recent progress and present their current notions about the LTD4 receptor signalling process.