Longitudinal relaxation related to intermolecular dipolar interactions was investigated by solution NMR. A novel pulse sequence with selective pre-saturation and selective excitation was designed for a highly polarized two-component spin system. A general analytical expression for the longitudinal relaxation was derived from a combination of the demagnetizing field theory and product operator formalism. The analytical expression suggests that the longitudinal relaxation is not a mono-exponential process. The predicted relaxation relationship is in excellent agreement with the experimental recovery curves.