The structural changes of hydrophilic polymer networks were studied by SANS. The size of heterogeneities in the network was measured for gels swollen in different amount of water. The type of cross-linking agent, degree of cross-linking and degree of ionisation were varied. The dependence of the size of heterogeneities on the degree of swelling shows a great difference for gels which exhibit strong polyelectrolyte properties (can swell hundreds of times) comparing with the ones which can swell less than 100 g/g. The interpretation of the observed phenomenon is based on the appearance of strong electrostatic interactions which are dominating in the gel-swelling mechanism at high degree of swelling.