We present a comparative study of Nernst effect and resistivity in underdoped samples of Bi2Sr2CuO 6+δ and La2−xSrxCuO4. 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.PACS numbers: 72.15. Jf, 74.25.Fy, 74.40.+k, 74.60.Ge Recently, the peculiar behavior of Nernst effect in copper oxide superconductors has become a subject of growing attention [1,2,3,4,5,6,7,8,9]. In conventional superconductors, Nernst effect, namely the transverse component of the thermopower in a magnetic field, is known to be associated with vortex movement [10]. However, Xu et al. [1] reported that in underdoped La 2−x Sr x CuO 4 a large residual signal persists in the normal state and well above T c . This observation has been confirmed in other families of cuprate superconductors including Bi 2 Sr 2 CuO 6+δ and YBa 2 Cu 3 O 6+y [2,3]. The normalstate Nernst signal is present over a wide doping range, but attenuates as one moves towards the superconductorinsulator boundary or the overdoped regime in the phase diagram [2]. Remarkably, the temperature scale up to which such an anomalous Nernst signal extends is about 130K, regardless of the particular system studied. Moreover, the Nernst signal is found to persist at magnetic fields as high as 30T both in overdoped [3] and in underdoped [4] compounds.The debate on the origin of this residual Nernst signal in the normal state of underdoped cuprates is still open and has stimulated a number of theoretical works during the past months. Fluctuations of the superconducting order parameter above T c are considered to be the most plausible explanation for such a signal. Ussishkin, Sondhi and Huse [5], for example, used a TimeDependent-Ginzburg-Landau(TDGL) approach. Within gaussian approximation and subtracting magnetization currents, they found a sizeable Nernst signal due to superconducting fluctuations. Weng and Muthukumar [7] have suggested that in a Resonant-Valence-Bond(RVB) picture, the coupling of spinon vortices to holons, can give rise to an enhanced Nernst effect above T c in the so-called spontaneous vortex phase. A different route has been taken by Kontani[6] who calculated Nernst coefficient beyond the Relaxation Time Approximation. By taking a self-consistent account of vertex corrections for currents, he found that the quasi-particle contribution to Nernst effect is no longer negligible in presence of antiferromagnetic and superconducting fluctuations. These studies point to the presence of strong superconducting fluctuations in the pseudogap regime and suggest that the superconducting transition is not a meanfield transition[11] but rather a vortex-antivortex unbinding type of transition [12]. It is ...