Two modes of electron gas injection in resonant tunnelling through GaAs/AlGaAs double-barrier heterostructures were revealed while studying their current-voltage characteristics. Examining peculiarities of the characteristics within the temperature range 4-350 K and under a high magnetic field, we were able to distinguish the contribution to resonant tunnelling of ballistic electrons mjected from a three-dimensional electron gas in the emitter contact and that of electrons injected from a two-dimensional electron gas in the accumulation layer formed near the emitter barrier.