We study how a local air plasma treatment affects the mechanical properties of polystyrene by performing indentation measurements on the polymer in the elastic and plastic regime. The local exposure to plasma was obtained by placing a shadow-mask with quadratic holes of 45 x 45 mu m(2) on top of the polymer substrate, providing uncovered (exposed to the plasma) and covered (protected from the plasma) areas. We have analyzed quantitatively the topography and the elastic-plastic properties of such a sample with atomic force microscopy (AFM) measurements, both before and after plasma treatment. To enhance the differences between covered and uncovered areas, the sample has been exposed to solvent vapor. This generates regions which are differently swollen. The quantitative investigation of the mechanical properties of the swollen sample for different solvent exposure times gives further insight into the changes of polystyrene mechanical properties caused by the plasma.