Abstract: Familiar mechanisms of metastable decay are tunneling and thermal activation. We show that periodically modulated systems may display a different decay mechanism, quantum activation. Here, decay occurs via quantum fluctuations induced diffusion over a quasi-energy barrier. We study quantum activation for nonlinear oscillators and show its unexpected features. The decay rate displays scaling behavior near critical parameter values where metastable vibrational states disappear. We also show that the tunnel splitting of symmetric and antisymmetric vibrational states oscillates and changes sign with varying parameters. The results impose limitations on the characteristics of bifurcation amplifiers used in quantum measurements.