Reversible deswelling of latex nanoparticles, induced by electrostatic repulsion after adding polyelectrolyte (polystyrenesulfonate, PSS) chains of the same charge to suspensions of the particles in deionized water, is shown using time-resolved fluorescence depolarization of fluorescent dyes (Lumogen 5 Yellow 083) impregnated within the nanoparticles, using methods of two-photon excitation. Shrinkage of the particles produced closer spacing between dyes, thus enhancing fluorescence quenching and homo energy transfer, which depolarizes fluorescence emission. Thus, it was observed that the higher the PSS concentration the more rapid decay of fluorescence depolarization on the subnanosecond time scale. When NaCl was subsequently added, screening caused the original particle size to be recovered. Tentatively, the diameter increase was roughly 7%. The spatial resolution provided by two-photon excitation implies that this method can be used to estimate the size changes of particles in situ in concentrated semiturbid solutions.