In this work, slightly charged thermo-responsive gels in the presence of salt at concentrations close to physiological conditions have been simulated within a coarse-grained model widely used in the last decade. These simulations allow differentiate charge and salt effects, which are antagonist and coupled in many real systems because the degree of ionization might depend on the electrolyte concentration. An analysis in terms of the different contributions to osmotic pressure is also presented, which highlights the role played by excluded volume effects. In addition, our results also permit us to test some predictions based on the ideal Donnan equilibrium, a common assumption made to justify the swelling behavior of gels and microgels in the presence of salt. More specifically, simulations show that, for the slightly charged gels simulated here, such an assumption overestimates the concentration of salt inside collapsed gels and underestimates the excess of osmotic pressure associated to the additional electrolyte. (c) 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 1403-1411