Heterocyclic Polymer Networks (HPN) prepared by trimerization of 1,6-hexamethylene diisocyanate (HMDI) in presence of diglycidyl ether of bisphenol A (DGEBA) and triethylene diamine in two different regimes were characterized by dielectric spectroscopy in the 10(-1)-10(5) Hz frequency range and in the 123 K-423 K temperature interval. The dielectric experiments reveal the existence of multiple composition dependent relaxation processes. The gamma process, appearing at low temperatures only in partially cured systems is related to the presence of free DGEBA. The beta process is associated to local motions of the carbonyl groups present in the isocyanurate and oxazolidone cycles. The alpha relaxation, which has its origin in the segmental motions above the glass transition temperature, tends to appear at higher temperatures with increasing network junction density and with the degree of curing. The fragility and cooperativity of the alpha relaxation is discussed in terms of recent relaxation approaches.