Using a method which is the electromagnetic analog of the scattering matrix formalism in the theory of nuclear reactions, two ``sum rules'' are derived for the frequency independent coefficients occurring in the admittance matrix relating the currents and voltages in a loss‐free microwave junction. These are sums, respectively, over the various modes of the guides entering the junction, and over the modes of the cavity comprising the junction (suitably defined). The results are used to estimate the effect on the admittance matrix of the higher modes, both of the guides, and of the junction proper.