The effects of 2-chloroethylphosphonic acid (Etlhrel), etlhylene, and sonie growth retardants oii sex expression of enlcumber plants (Cuctumis sativtrs L.) were investigated, with the use of a monoeciouis ctultivar (Improved Long Green) w-hichi has a strong tendency toward nmaleness.Ethrel caused iincreased femaleniess whlen applied at 50 milligram-s per liter at the first to the third leaf stage, but when applied at the cotyledon stage it was ineffective. The later the timne of application, the hiiglher the niode at which the first female flower appeared. Thie total number of female flowers was abouit the same regardless of applicationi time. A mixture of gibberellins A4 anld A7 caused maleness, and Ethrel caused femaleness. hlowever, whent applied in combination at the first leaf stage the initeraction was not significant. It seems, therefore, thiat Etlirel and gibberellins are not antagoniistic buit rather hav-e (lifferent sites of action, although they have opposing effects oni sex expressioni.Ethylene caused femaleness but was far less effective than Ethrel, Alar (N-dimiietihylamiinosucciinamic acid), CCC ((2-chloroethyl)trimethylamnmonitum clhloride), Phosphon D(2, 4-dichlorobenzyl-tributtylplhosphlotnitnm chloride), alnd abscisic acid did niot affect sex expressioni of cucumiiiber.Cucumber flower buds, which differentiate in leaf axils of main shoots, are bisexual in their early developmental stages. Later they develop into either staminate (male) or pistillate (female) flowers (1,7,12). In monoecious varieties male flowers differentiate at the lower nodes, followed by female flowers at the higher nodes. The number of nodes to the first female flower and total number of female flowers are both reliable indices of sex expression.Although sex expression in cucumber plants is determined genetically, it is modified by several environmental factors. High nitrogen, short days, low light intensity, and low night temperature are among the factors which favor femaleness. Reverse conditions tend to cause maleness (11).The concept that sex expression of cucumber plants may be regulated by a balance between native auxins and gibberellins (2) is supported by two types of evidence. Applied auxins, especially a-naphthaleneacetic acid, induce femaleness (8,13,15), whereas gibberellins induce maleness (8,23,27 Recently it was demonstrated that in some plant responses auxin may exert its effect through ethylene evolution (29) and that gibberellins have generally an opposing effect to ethylene (28). Since it has long been known that unsaturated hydrocarbons (17, 18,19) induce femaleness in cucumber, it appears that ethylene, the most potent unsaturated hydrocarbon affecting plants (4), may be the active factor for inducing femaleness.Recently it was found that Ethrel2, which releases ethylene in the presence of plant tissues (31), is remarkably effective in increasing femaleness in cucumber (14,16,24,25,30). The experiments reported here were initiated to study the effects of ethylene, Ethrel, gibberellins, growth r...