Abstract: A greenhouse experiment was carried out to investigate the effects of different sizes of normal seed tubers (35-45, 45-55 and 55-65 mm) and various growth regulators (gibberellic acid: 2.5 and 5 mg L-1; chlormequat: 10 and 100 mg L-1; included two controls: cut seed tubers and uncut seed tubers) on the dormancy, sprout behaviour, growth vigour and quality of tubers of different potato cultivars Frieslander, Marfona and Sante. Size of tuber, growth regulator and cultivar showed a highly significant effect on the sprouting behaviour. Highest number of sprouts with longest length and highest weight were produced by the size 55-65 mm. Highest dose of GA resulted highest number of sprouts and uncut control produced lowest number of sprouts. Frieslander produced higher number of sprouts and also more weight of sprouts than other cultivars. All parameters of vigour were highly significantly affected by cultivars and growth regulators while seed tuber size only affected number of stems. There were many significant two-way interactions between seed tuber size, cultivar or growth regulator application.