The main aim of this study was to test if the seed coat is a barrier to restrict juglone's entrance into the seed using intact and coatless seeds of cucumber (Cucumis sativus cv. Beith Alpha). Elongation, fresh and dry weights of cucumber seedlings were decreased significantly by juglone in both intact and coatless seeds. Protein content in cotyledons of cucumber seedling was also decreased by juglone. The result do not prove the seed coat barrier hypothesis Because juglone's inhibitory effect on seedling growth and protein content was not so much different in coatless seeds than was in intact seeds. However, in the present study, catechol and dopa oxidizing activities of polyphenol oxidase enzyme were increased by juglone in both seedlings. The increase in the activities of this enzyme was attributed to a reaction against to allelochemical stress produced by juglone.