Young onion plants were grown in glass tubes under short daylength non-inductive for bulbing. Ethrel was added to the growth medium and caused swelling of leaf-bases, bulb initiation and a pronounced increase in the bulbing index. Swelling of leaf-bases was also observed in Ethrel treated plants of leek. In a replicated field experiment seedlings of 5 cultivars were sprayed once or several times with Ethrel at rates of 500, 1,000, 5,000 and 10,000 ppm. Except for one very early cultivar, which showed bulbing due to natural daylength, the treatments resulted in early bulb initiation and in a greatly increased rate of bulbing. Rates of 5,000 and 10,000 ppm Ethrel were more effective than lower concentrations but they also caused retarded leaf growth and a decrease in the final size of bulbs. The possible role of ethylene in bulbing of onion is discussed.