We have exploited angle-resolved photoelectron spectroscopy to study changes in the quasiparticle excitations of CuO 2 planes as a function of the carrier density, in optimally doped and underdoped Bi 2 Sr 2 CaCuO 8 + x and in Sr 2 CuO 2 Cl 2 . We have observed large changes in the E versus k dispersion of the quasiparticle bands, and in the topology of the Fermi surface away from optimal doping. We have also found that the carrier density has a nontrivial effect on the symmetry properties of these states.