  We present a comparative study of Nernst effect and resistivity in underdoped
samples of Bi$_2$Sr$_2$CuO$_{6+\delta}$ and La$_{2-x}$Sr$_{x}$CuO$_4$. The
Nernst effect presents a peak in a region of the H-T diagram where resistivity
shows a non-metallic temperature dependence. Our results illustrate that the
mechanism of dissipation in the mixed state of underdoped cuprates is poorly
understood. Large quantum superconducting fluctuations and vanishing vortex
viscosity are among suggested explanations for an enhanced Nernst signal close
to the superconductor-insulator boundary.
