An experimental investigation is presented for the substrate current (holes) appearing in n-channel field-effect transistors having SiO2 as their gate insulator. In these experiments, the gate is biased by a high and positive voltage, causing an electron current to be injected from the device channel into the oxide. This current is accompanied by the substrate current whose origin is not clear. The experiments were performed by application of short pulses (400 μsec) to the gate. It is shown that the substrate current is too large to be explained by simple electron tunneling from the silicon valence band into the oxide. Temperature-dependence measurements, down to 20 K, show that some of the data are not consistent with models for hole transport from the oxide into the silicon valence band. It is argued that the substrate current may be related to the energy loss experienced by hot electrons as they traverse the oxide. It is further argued that the same mechanism responsible for the substrate current may produce positive charge at the injecting electrode and thus lead to breakdown in thin oxide devices.