While ethylene is suspected to be one of the many factors that play a role in rooting, some studies have found that ethylene can promote rooting, while others show it to inhibit this process or to have no effect. Using seedlings of sunflower (Helianthus annuus L. cv. Dahlgren 131) we carried out observations on the rates of ethylene production and the levels of the ethylene precursor, 1-aminocyclopropane-l-car-boxylic acid (ACC), and ACC conjugate, l-(malonylamino)cyclopropane-l-car-boxylic acid (MACC), during the process of root initiation. The changes in these substances in the base of the hypocotyls (the portion that produces roots) were compared to the changes that occurred in the top of the hypocotyls (non-rooting portion). We also supplied a number of presumptive inhibitors of ethylene biosynthesis and inhibitors of ethylene action for short periods during the early and critical stages of root formation. Their effects on ethylene action, synthesis and rooting were examined. We conclude that the wound-induced increase in ethylene, seen within 3 h of production of the cuttings, is a key stimulatory factor in the formation of root primordia. When this increase in ethylene is localized in the lower portion of the hypocotyl, there is a promotion of rooting. On the other hand, higher concentrations in the top of the hypocotyls (as compared to the bottom) may inhibit rooting.