The dispersion relations for form factors are studied in perturbation theory. In particular, the relations are proved for a realistic nucleon in the form usually assumed in literature. It is demonstrated, however, that in more general cases the mass spectrum in the dispersion relations depends on the masses of the contributing particles in a peculiar way, which is different from the simple relation hitherto believed. Such a situation arises, for example, in the case of some hyperon models and bound systems such as the deuteron. A formula concerning the final state interaction to be used in conjunction with the dispersion relations is also derived from the invariance of the theory under space-time reversal.