Laboratory measurements of seismic velocities in upper mantle peridotite can be used to constrain the thermal state in the los velocity zone. The measurements, carried out at high temperature and high confining pressure, can be extrapolated to upper mantle conditions. We compare laboratory velocities with seismic observations, and estimate temperatures and partial melt fractions in the low velocity zone. The comparison gives 1310¿C and 7 vol.% partial melt at 45km depth in 0-5 my asthenosphere beneath the Iceland Plateau, and 1450¿C and 3 vol.% partial melt at 85km depth in 0-20 my asthenosphere beneath the Pacific Ocean. The laboratory results show that up to a 6% velocity zone (relative to the lithospheric velocity) does not necessarily require the existence of partial melt. This implies that partial melt may not exist in the low velocity zone older than ~5 my and ~50 my in the Iceland Plateau and the Pacific Ocean, respectively. ¿ American Geophysical Union 1988