Arborescent graft or comb-burst polymers are highly branched, high molecular weight polymers that are constructed from linear polymer chains by a cascade grafting process. The arborescent graft polystyrenes studied here were prepared by a "graft on graft" synthetic strategy employing anionically prepared branches, which led to well-defined branched polymers with controlled branch lengths and low polydispersities. The dynamic mechanical behavior of these arborescent graft polystyrenes in the melt was studied as a function of branch length and grafting generation. Terminal relaxation times and zero-shear viscosities of the lower-generation arborescent graft polymers increase progressively with the molecular weight of the branches. Even in the case of the highly branched molecules, all samples demonstrated viscous now behavior at small shear rates. The frequency dependence of the dynamic moduli changed with increasing number of grafting generations showing the features of a cross-linking polymer at the gel point. The most highly branched arborescent graft polymers display a frequency dependence similar to those of cross-linked networks and microgels.