<p>A generalized method has been developed to describe the time dependences of the orientation factor for Förster resonance dipole–dipole energy transfer (FRET) between fluorescent labels associated with biomolecules taking into account internal rotational mobility. The method consists of representing the orientation factor in terms of tensor orientational correlation functions for vectors of transition dipole moments of donor and acceptor, which makes it possible to use well-known proven models of orientational relaxation to describe rotational diffusion. The time dependences of the probability of the energy-transfer process on the opening angles of internal rotational diffusion cones, the orientation of these cones on the surface of the biomolecule, and the rotational diffusion rates were calculated. The ranges of angular parameters at which the temporal kinetics had the most dramatic changes during a transition from the initial stationary values to the dynamic limit were determined. It has been demonstrated that the internal rotational diffusion of the donor and acceptor under certain conditions can both decrease and increase the probability of energy transfer over time.</p>