An equation is derived that describes how interelectronic interactions determine the time variation of one specific order parameter in a pure superconductor. The equation is derived from the equations of motion obeyed by the non-equilibrium one particle Greens function in the presence of time dependent fields and interactions. These are related to the findings of L. P. Gorkov Sov. Phys. JETP, 7505 1958, 91364 1962, and 10998 1960.