We describe a molecular theory for solvent dynamics which provides a method for calculating the dynamic structure factors of molecular fluids. The theory is based on the generalized Langevin equation and on the interaction-site model for molecular liquids. A simple approximation scheme for the memory function is developed by generalizing the conventional one which has been successfully applied to monatomic liquids. The theory is applied to the calculation of longitudinal current spectra of a diatomic dipolar liquid, and collective excitations in this solvent are studied. The paper clarifies how these excitations originate from the translational and rotational motions of molecules.