Tuberous roots of Viguiera discolor accumulate up to 80% of their dry mass as fructans. The distribution pattern of oligomers suggests the predominance of anabolic reactions at the beginning of dormancy, when a continuous series of fructans between sucrose and higher polymers is present. This paper describes the synthesis in vitro of fructans of high molecular mass by enzyme extracts prepared from growing tuberous roots of V. discolor at the beginning of dormancy. Sucrose[ratio ]sucrose fructosyl transferase activity was characterized by the synthesis of 1-kestose from sucrose and fructan[ratio ]fructan fructosyl transferase activity by the production of nystose from 1-kestose. The optimal temperature for both activities was 30°C and the optimal pH range was between 5·5 and 6·0. The apparent Km for sucrose with respect to 1-kestose synthesis was 173 mol m−3, whereas the Km for nystose synthesis from 1-kestose was much higher (592 mol m−3). Long incubation periods (up to 264 h) with sucrose, 1-kestose or nystose produced fructans of high molecular mass in vitro that resembled those found in vivo. Our data show that the higher the molecular mass of the substrate, the higher the average molecular mass of the product that was formed.