Pressure (0-19 kbar) and temperature (4-300 K) dependent EPR study of Sodium-Electro-Sodalite (SES) is presented. SES, which consists or a bcc sub-lattice of F-centers supported by a zeolite-like framework, is known to be a Mott insulator at room temperature. On cooling, SES undergoes an AF transition at 48+/-2 K providing the first example of an s-electron antiferromagnet. We find that the width of the EPR resonance above T-N is influenced not only by a strong exchange interaction, but also by a fast spin-lattice relaxation. Also, with increasing pressure, T-N decreases linearly and extrapolates to 0 K at about 65 kbar. The reason for this seemingly unexpected behavior is briefly discussed.