The dielectric breakdown of ultrathin SiO2 films has been studied by subjecting chromium-gated metal/oxide/semiconductor structures to wear-out tests. While in constant-current stress tests only one breakdown is observed as a sudden voltage drop, in constant-voltage stress experiments, several breakdown events can be registered as abrupt current increments. No experimental evidence for the occurence of important thermal effects has been found, and the breakdown has been interpreted as a reversible switching between two oxide conduction states of very different conductivities. During our constant-voltage tests, breakdown and "anti-breakdown" events were observed as abrupt changes between well defined current levels which correspond to the presence of a variable number of breakdown spots. The "effective" number of breakdown spots increases with the time of stress, as has been demonstrated by the evolution of the after-breakdown current-voltage characteristic. All these results are understood if one considers that the breakdown is a purely electronic phenomenon related to the degradation of the insulator network. Assuming that the main effect of the degradation of the SiO2 microscopic structure is the generation of deep electron traps near the injecting interface, trap-assisted resonant tunnelling seems to be a reasonable hypothesis for the breakdown triggering mechanism. Nevertheless, the actual nature of this conduction mechanism is still an open question. © 1991.