Quantum-dot structures can exhibit bistability between two conduction states connected with S-shaped current-voltage characteristics. In this work, a dynamical model for the electric transport in this structure is proposed. Switching times of about 5 ps at threshold are found for small sample capacitances. On the other hand, it seems to take more than 1 ns before the steady state is reached when the device switches from the high to the low conducting state. In addition, relaxation-type oscillations are predicted under current-controlled conditions if the capacitance of the structure is large enough.