One hundred and ten measurements of thermal conductivty have been made on samples over a 1900-m depth interval in the Reydarfjordur borehole comprising largely basaltic flows and intrusive. The mean conductivity of both flows and intrusive increases with depth by about 25%, reaching a maximum value of about 1.9 W m-1 ¿C-1. This increase results from the growth of secondary minerals which progressively fill voids with depth and in some cases replace phases having lower values of thermal conductivity. The conductive heat flow between 650 m and the surface is about 100 mW m-2. Below 650 m the conductive heat flow appears to be less than half the near-surface value, and there is strong evidence that in this depth interval the thermal structure is modified by hydrothermal circulation.