Two transgenic potato lines, T1 and T2, expressing the trehalose-6-phosphate synthase (TPS1) gene of yeast were isolated. In our experimental approach, we applied two novelties, namely the fusion of the drought-inducible promoter StDS2 to TPS1 and a marker-free transformation method. In contrast to the expected drought-induced expression, only a very low constitutive TPS1 expression was detected in the transgenic lines, probably due to chromosomal position effects. The observed expression pattern, however, was sufficient to alter the drought response of plants. Detached leaves of T1 and T2 showed an 8 h delay in wilting compared to the non-transformed control. Potted plants of T1 and T2 kept water 6 days longer than control plants and maintained high stomatal conductance and a satisfactory rate of net photosynthesis. During drought treatment, CO2 assimilation rate measured at saturating CO2 level was maintained at maximum level for 6-9 days in transgenic plants while it decreased rapidly after 3 days in the wild type plants. Under optimal growth conditions, lower CO2 fixation was detected in the transgenic than in the control plants. Stomatal densities of T1 and T2 leaves were reduced by 30-40%. This may have contributed to the lower CO2 fixation rate and altered drought response.