Small angle x-ray scattering was used to follow changes in the conformation and interactions of nucleosome core particles (NCP) as a function of the monovalent salt concentration C(s). The maximal extension (D(max)) of the NCP (145 +/- 3-bp DNA) increases from 137 +/- 5 A to 165 +/- 5 A when C(s) rises from 10 to 50 mM and remains constant with further increases of C(s) up to 200 mM. In view of the very weak increase of the R(g) value in the same C(s) range, we attribute this D(max) variation to tail extension, a proposal confirmed by simulations of the entire I(q) curves, considering an ideal solution of particles with tails either condensed or extended. This tail extension is observed at higher salt values when particles contain longer DNA fragments (165 +/- 10 bp). The maximal extension of the tails always coincides with the screening of repulsive interactions between particles. The second virial coefficient becomes smaller than the hard sphere virial coefficient and eventually becomes negative (net attractive interactions) for NCP(145). Addition of salt simultaneously screens Coulombic repulsive interactions between NCP and Coulombic attractive interactions between tails and DNA inside the NCP. We discuss how the coupling of these two phenomena may be of biological relevance.