Cytokinin (CK) levels in cotyledons of Cucurbita pepo L. (zucchini) were investigated through the processes of postgermination, greening, natural senescence and subsequent rejuvenation. The concentrations of the physiologically active CK bases, ribosides and nucleotides, as well as the cis-isomers of zeatin derivatives, decreased between the first and fifth weeks of cultivation under controlled light conditions. At the same time, the levels of storage CK O-glucosides and physiologically inactive CK 7-and 9-glucosides increased with senescence. With plant decapitation and subsequent cotyledon rejuvenation, not only the chlorophyll content but also the levels of physiologically active CKs, nucleotides and cis-zeatin derivatives increased. The levels of O-glucosides, however, decreased. When 1-week-old seedlings were transferred to the dark, there was a progressive reduction in cotyledon chlorophyll content, deterioration of chloroplast ultrastructure and a decrease in physiologically active CKs and their nucleotides. In contrast with natural senescence, the storage CK O-glucosides decreased under dark conditions, suggesting different metabolic regulation of endogenous CK levels during natural and dark-induced senescence of zucchini cotyledons. The chlorophyll loss of dark-treated cotyledons could be partially reversed, even after 5 days, with return to light conditions. During this recovery, physiologically active CKs and their nucleotides again increased, whereas the storage CK O-glucosides and cis-zeatins decreased. The present results suggest that dark-induced destruction and subsequent restoration of chloroplasts during light shifts are controlled by changes in the levels of physiologically active CKs and their nucleotides.Abbreviations -CK, cytokinin; DHZ, dihydrozeatin; DHZ7G, dihydrozeatin 7-glucoside; DHZ9G, dihydrozeatin 9-glucoside; DHZR, dihydrozeatin 9-riboside; DHZRMP, dihydrozeatin 9-riboside-5 0 -monophosphate; DHZROG, dihydrozeatin 9-riboside O-glucoside; FW, fresh weight; iP, N 6 -(D 2 -isopentenyl)adenine; iP7G, N 6 -(D 2 -isopentenyl)adenine 7-glucoside; iP9G, N 6 -(D 2 -isopentenyl)adenine 9-glucoside; iPR, N 6 -(D 2 -isopentenyl)adenine 9-riboside; iPRMP, N 6 -(D 2 -isopentenyl)adenine 9-riboside-5 0