Whey proteins are globular, heat-sensitive proteins. The gel structure, the formation of this structure, and the rheological properties of particulate whey protein isolate (WPI) gels have been investigated. On increasing the NaCl concentration, the permeability of the WPI gels increased, indicating a coarsening of the gel structure, confirmed by confocal scanning laser microscopy pictures. Only a part of the total amount of protein present contributed to the gel network at the gel point (the primary spatial structure). Large variations were observed in the amount of aggregated material at the gel point (and thus the primary spatial structure) as a function of NaCl concentration, due to differences in the kinetics of the denaturation/aggregation process. After the gel point more protein is incorporated in the gel network by "thickening" the strands in the gel and "decorating" the pores in the gel, apparently without changing the gross spatial structure. Power law behavior was found for the permeability dependence of aged gels on the amount of aggregated material at the gel point. For various salt concentrations the curves coincided to one master curve. This power law behavior is consistent with a primary spatial structure of fractal flocs with a fractal dimensionality of 2.4. The elastic modulus is remarkably related (via a power law) with the total amount of protein contributing to the gel network, in contrast to permeability.