Measurements of differential scanning calorimetry, dynamic mechanical analysis, and dilatometry have been performed in heterocyclic polymer networks (HPNs), whose effective network density has been gradually varied keeping the overall chemical structure essentially unchanged. Evidence of a growing intermolecular cooperativity for a local relaxation motion is offered by the investigation of the subglass mechanical beta-relaxation, whose frequency factor and apparent activation energy strongly increase with increasing cross-linking density from about 10(15) s(-1) 44.2 kJ/mol to about 10(19) s(-1) and 69.8 kJ/mol. The analysis of the characteristics of the mechanical beta-relaxation suggests the existence of cooperative transitions of consecutive relaxing units, mainly driven by the crank-shaft motion of the network junctions between the isocyanurate heterocycles building up the structure. Comparison with previous results concerning the dielectric beta-relaxation evidence a less cooperative character and a different microscopic origin for the conformational transitions driving the dielectric process. Moreover, a decrease in the thermodynamic fragility with increasing network density has been revealed, which contrasts with the increasing dynamic fragility, as measured by dielectric and mechanical probes.