Hypocotyl hooks have been shown to influence greening in excised cucumber (Cucumis sativus) cotyledons. The properties of the lag phase are greatly affected by the presence or absence of the hook tissue. A 45-second light pretreatment followed by 4 hours of darkness is sufficient to remove the lag phase from cotyledons with hooks, while hookless cotyledons require 2 hours of continuous illumination followed by 1 hour of dark incubation to break the lag phase. The effect of hooks on cotyledon greening is enhanced if the hooks are shielded from light. Cutting off the hooks after lag phase removal caused a marked decrease in chlorophyll accumulation in the cotyledons. These observations may indicate that the hypocotyl hooks produce a substance or substances needed in the greening process, which are translocated to the cotyledons. Indoleacetic acid, abscisic acid, gibberellin A(3), 6-benzylamino purine and delta-aminolevulinic acid do not show any activity; on the other hand, ethylene appears to replace partially the hypocotyl hooks.