Starting from the inhomogeneous quantum-mechanical Maxwell equations we give a rigorous quantum-field theoretical derivation of the Langevin equations for a high-Qcavity field and specify the conditions of validity without borrowing from the standard relaxational treatment. In particular, we show that the use of Langevin equations means a loss of information about field changes during times of the order of the time of flight of a light signal through the cavity, which may be crucial in picosecond and femtosecond spectroscopy.