Abstract: Measurement of the response of the flux-line lattice in NbSe2 and La1.825Sr0.075CuO4 show important differences between these two materials. In particular, we have studied the magnetic-field and angular dependence of the response of a high-Q mechanical oscillator in fields of up to 1 T. The features seen in NbSe2 seem to be well explained in terms of a change in the pinning regime, usually termed the “peak effect” in the critical current, using the collective-pinning model of Larkin and Ovchivnikov within Ginzburg-Landau anisotropic theory. On the other hand the behavior found in the high-Tc material LSCO seems to fall naturally into a description which takes into account the possibility of phase transitions in the vortex lattice and the quasi-two-dimensional character of the superconductivity.