We consider a quantum dot coupled to two leads in the presence of oscillating external fields applied to the dot and the two leads. Using the nonequilibrium-Green-function method, the formula for the transient transmission probability (TTP) is derived. The numerical studies reveal the following facts. When ( is the full width of the resonant state), no visible resonant tunnelling behaviour occurs, and strongly depends on the time at which the electron enters the dot, t, and that at which it exits from it, ; when , the resonant tunnelling becomes striking, and depends only weakly on t and . The conventional transmission probability , defined as the limit as and , has a very rich spectrum and exhibits a simultaneous quenching of some channels. On the basis of the properties of , the condition for the occurrence of an electron-photon pump has also been discussed.