The effects of daylength and its extension (Exp.1) and a light interruption at night (Night Break, NB : Exp.2) at different times on shoot growth and flower bud differentiation in potted 'Pione' grape (Vitis vinifera x V. labruscana) were investigated in a greenhouse (forced conditions). Both experiments were conducted for 13 weeks beginning at the end of December when the natural daylength corresponded to about 9hr and 50 min. The primary shoot was pinched to 15 nodes and all lateral shoots were repeatedly pinched, leaving just one leaf at the node. In Exp.1 an additional lighting was supplied with a homolux lamp as follows : 6 hr after sunset, 6 hr before sunrise, 3 hr after sunset and 3 hr before sunrise, and from sunset to sunrise. The elongation of the primary shoot, the number of leaves on lateral shoots per main shoot and leaf area per shoot were significantly increased when the daylength was extended as compared to the control vines grown under natural daylight. The mean number of bunch primordia per lateral bud in vines exposed to lamp increased three to four-fold compared with the control. No significant difference in shoot growth and flower bud differentiation was observed among the three plots for which daylength was prolonged to 16 hr. Night interruption for 2 hr from 8 p.m., 11 p.m., and 2 a.m. with a homolux lamp (Exp.2) promoted shoot growth compared to the control vines ; the number of bunch primordia was almost the same. There was no significant difference in shoot growth and the flower bud differentiation among the three night break treatments. Among the plots the most vigorous shoot growth and the largest number of bunch primordia per bud occurred in vines exposed to constant (24 hr) light.