We obtained 17 successful 12.5 m heat flow measurements on Anomaly 13 east of the Reykjanes ridge. All temperature gradients were linear below 7.5 m but showed a large increase toward the sediment surface in the upper 5m. From the linear gradients and thermal conductivities measured on the sediment recovered in the cores we computed a mean flow for Anomaly 13 (35 m.y. B.P.) of 1.98¿0.24 μcal/cm2 s (84.2¿10.5 mW/m2). Thirteen of the stations were taken in a detailed survey of a well-sedimented area at 56¿N, 29¿30'W. These measurements have a mean of 2.01¿0.25 μcal/cm2 s (84.2¿10.5 mW/m2). Almost all of the scatter in the data in this area can be accounted for by a combination of the refractive effect of the high thermal conductivity of the basement rocks and instrumental error. However, it is not possible from the present data set to rule out hydrothermal circulation in a 'permeable' basement as an explanation of the variation in the data. the mean value for the survey area is in excellent agreement with the relation between heat flow and the reciprocal of the square root of age predicted for oceanic crust less than 120 m.y. old. The sharp decrease in the temperature in the upper 5 m of sediment is most plausibly accounted for by a yearly periodic temperature change of an amplitude of 0.40¿C in the Norwegian Sea overflow water. We suggest that the major temperature change occurs in late spring and reaches the area of our survey by early summer.