The effect of time-periodical blowing through a spanwise slot on the near wall turbulence characteristics is investigated. The blowing velocity changes in a cyclic manner from 0 to 5 wall units. The frequency of the oscillations is equal to the median frequency of the wall turbulence. The flow field near the blowing slot is partly relaminarized during the acceleration phase of the injection velocity until 40 wall units downstream. The imposed unsteadiness is confined into the buffer layer and the time mean structural parameters under unsteady blowing are found close to those of an isotropic turbulence in this zone. The relaminarized phase is unstable and gives place to a coherent spanwise structure which, in return, increases the shear from 80 to 300 wall units downstream of the slot in a predictible way.