[en] Investigations of the start-up phase in a large tokamak are made using a zero-dimensional, time-dependent model which describes the ionization of the neutral gas in terms of coupled mass and energy conservation equations for five components - H₂⁰, H₂⁺, H₁⁰, H₁⁺ and electrons. These equations are coupled to an equivalent external OH circuit of the tokamak and the plasma behaviour during start-up for various discharge conditions and models for loss mechanisms has been studied. A simple model for the effect of transverse magnetic fields on the discharge formation has also been included to investigate the effect of a divertor field and also magnetic limiter operation. (orig.) 891 GG/orig. 892 HIS