We have studied the dynamic response of Pb thin films with a square array of antidots by means of ac susceptibility vΓ°T ; H Γ measurements. For low amplitudes h of the ac excitation, the field dependence of the ac susceptibility exhibits a series of distinctive features associated with the presence of different dynamic regimes. For fields below a certain characteristic field H s , vortices strongly pinned by the antidots perform an oscillatory movement inside the pinning potential induced by the external ac drive. For fields between H s and H p , interstitial vortices, performing an intra-valley motion in the weak potential wells produced by the repulsive interaction of their pinned neighbors, dominate the ac response. For H p < H < H t , the interstitial vortices are driven out of their pinning sites while vortices in the antidots remain trapped. For higher fields all vortices are set in motion and a critical state develops. We show that the characteristic h-independent field H s allows one to clearly identify the maximum number of flux quanta per hole. Unlike H s , the fields H p and H t , signaling the onset of motion of interstitial vortices and vortices trapped by the antidots, respectively, strongly depend on h.