Endogenous cytokinins (CK) were measured in Cucurbita pepo L. (zucchini) cotyledons after spraying 7-day-old seedlings with methyl jasmonate (MeJA) to examine whether the senescence-promoting action of MeJA is associated with changes in metabolic regulation of CK levels. MeJA promoted senescence as estimated by the loss of chlorophyll and injury to chloroplast ultrastructure. This was already detected one day after treatment. The contents of physiologically active CKs determined by high-performance liquid chromatography-mass spectrometry (CK bases: trans-zeatin, dihydrozeatin and isopentenyladenine and their ribosides), especially trans-zeatin and its riboside declined considerably. CK nucleotides and physiologically inactive CK 7- and 9-glucosides were also markedly decreased whereas the content of storage CK O-glucosides as well as cis-isomers of zeatin declined only slightly. Chlorophyll content and the normal chloroplast structure recovered 5 days after the MeJa treatment. The process of photosynthetic apparatus recovery correlated with an increased concentration of physiologically active CKs, especially Z. In contrast, the content of CK nucleotides and storage CK O-glucosides decreased further. We suggest that the ability of MeJA to promote some aspects of senescence in intact zucchini cotyledons is at least partially due to down-regulation of endogenous CK levels and their interconversion between active and inactive forms.