Average heat flow values on land in western Europe are about 2 h.f.u., whereas recently acquired surface-ship measurements and the downhole heat flow determinations at D.S.D.P. Site 402 indicate that the heat flow is only about one-half of this value over the continental slope and rise of the adjacent northern Bay of Biscay margin in the vicinity of the D.S.D.P. Sites 400-402. Assuming that both the margin and the adjacent continental area are close to steady state thermal conditions, we suggest that the observed heat flow contrast reflects different radioactive crustal contributions to the surface heat flow in the two areas. Under the margin, the crust is thinner and would have a smaller radioactive contribution, the decrease in the contribution being related to the nature of the crustal thinning process under the margin. The heat flow data are compatible with a model of crustal thinning that considers this to be mainly a result of mechanical deformation of the crust through extension.