We analyze the formation and decay properties of nucleon resonances formed in heavy-ion collisions at 1--2 GeV/[ital u] within a microscopic transport approach. In case of Au+Au reactions the density of [Delta] resonances reaches 0.15 fm[sup [minus]3] in the central cell for a time period of the order of 10 fm/[ital c] such that one can legitimately speak about [ital resonance] [ital matter]. The lifetime of the [Delta]'s is found to be shortened at high density by only 20% due to the in-medium channel [Delta]+[ital N][r arrow][ital N]+[ital N].