Half-fronds of Spirodela polyrrhiza accumulated high level of endogenous serine when they had been cultivated under long-day conditions and on a cytokinin-free medium for 4 days, after which the irreversible senescence processwas triggered. When 1 M6-benzyl adenine (6-BA) was included in the medium, the accumulation of endogenous serine was significantly inhibited, and at the same time the biological activity of half-fronds was increased. Application of exogenous l-serine into medium at 1mM initiated senescence of half-fronds, as well as intact plants, under the conditions of long-day and darkness. Application of 6-BA into the medium with l-serine postponed the appearance of serine level peak and senescence syndrome, although the senescence process was not prevented. It was also observed that half-fronds cultivated in the darkness on a medium with only inorganic nutrients had a very low level of endogenous serine and maintained their biological activity for up to 2 months. These data shows that high level of endogenous l-serine can initiate senescence in Spirodela polyrriza and cytokinin can inhibit the accumulation of endogenous l-serine. The possible reason for the accumulation of endogenous serine under long-day conditions was investigated. The activity of serine glyoxylate aminotransferase (SGAT), which catalyzes the conversion of l-serine to hydroxypyruvate, was found to decrease gradually in half-fronds during cultivation under long-day conditions, but to be maintained by 6-BA application. Northern blot analysis was used to detect the changes in the level of transcript encoding SAGT, and results showed that the expression of this gene was up-regulated by cytokinin but down-regulated during senescence.