In young plants of Salix pentandra, a temperate zone deciduous woody species, elongation growth ceases and a terminal bud is formed at day lengths shorter than a critical length. This is the first step in dormancy development, making survival under harsh winter conditions possible. Early studies strongly indicate that gibberellin is involved in the photoperiodic control of bud set and bud break. GA(1) action was studied by application under short days to plants where cessation of shoot elongation had occurred, followed by subsequent anatomic investigations of shoot tips. Under short days the frequency of cell division decreased rapidly along with the earlier observed decrease in GA(1) levels. Application of GA(1) to short-day-induced terminal buds rapidly stimulated cell division in apices several days before visible shoot elongation in response to this treatment was observed. One day after GA(1) application a fourfold increase in cell division frequency in apices was observed, increasing to a maximum of sevenfold 2 days after application. Long-day treatment leading to induction of bud break after about 4-6 days was followed by slowly increasing frequency of cell divisions. In earlier studies of this species, short days and gibberellins had no effect on cell elongation. These data show that increased GA(1) content, by application or long-day treatment, results in increased frequency of mitosis. This strongly indicates that GA(1) affects stem elongation in connection with bud set and bud break primarily by affecting cell divisions in subapical tissues.