The linear swelling ratio a and the effective network chain length N of a series of poly(N,N-dimethylacrylamide) (PDMAAm) hydrogels were investigated as a function of the gel preparation concentration v(2)(0). PDMAAm hydrogels were prepared at a fixed cross-linker ratio but at various initial monomer concentrations. It was found that alpha is not a monotonic function of v(2)(0). As v(2)(0) is increased, alpha first decreases up to about v(2)(0) = 0.10 and remains constant in a narrow range of v(2)(0), but then it increases continuously. The v(2)(0)-dependence of alpha is due to the variation of the network chain length N depending on the gel preparation concentration. In the range of v(2)(0), below 0.1, N follows the scaling relationship N approximate to (v(2)(o))(-2), while at higher concentrations, N varies only slightly with v(2)(o). The increase of alpha with N obeys the relation alpha approximate to (v(2)(o)N)(1/5), as predicted by the Flory-Rehner theory. (C) 2005 Elsevier Ltd. All rights reserved.