The question considered is the effect on the canonical commutation rules of mass renormalization in quantum etectrodynamics. This is investigated by considering the quantum Langevin equation for a charged quantum oscillator interacting with the radiation field via dipole interaction. Explicit expressions for the commutator are obtained for finite cutoff frequency as well as the large-cutoff limit. The surprising result obtained is that the exactly equal-time (canonical) commutator involves the bare mass, while the corresponding commutator for infinitesimally displaced times involves the renormalized mass.