Bottom-hole temperatures and lithologic information from 167 released wells between 52° and 58°N in the British sector of the North Sea have been studied and measurements of thermal conductivity have been conducted on many samples from all the main stratigraphic units. Although large errors may be associated with the temperature information, it is sufficiently reliable to establish that heat flow is not conserved with depth over much of the western North Sea. In the Central graben and Anglo-Dutch basin, heat flow in the upper 1-2 km (3,300-6,600 ft) is around 50 mW m^-2 increasing downward to about 100 mW m^-2. Farther west, on the Mid-North Sea high and on the East Midlands shelf, the situation is reversed with shallow heat flows of about 80 mW m^-2, decreasing downward to about 50 mW m^-2. These variations are not readily explained either by heat-flow refraction or by tectonic activity. They are most probably attributable to the deep and continuing circulation of water. The circulation may well be on a very large scale and must be largely fault controlled.