We report 48 new heat flow measurements on the western flank of the East Pacific Rise at 21 ¿N. The stations were taken in 1 to 30 m of sediment on 0.4 to 1.4 m.y. old crust. The low average measured value of ≥173 mW/2 (including 12 tilted minimum values) is about 1/3 the mean heat flow predicted for crust of this age range by cooling plate theory. This may be partly due to a bias of measurement locations in sediment ponds. Nevertheless, 4-km running averages of measured heat flows, projected normally to the axis, display on oscillatory trend suggestive of cellular hydrothermal circulation in the basement. We sampled no more than one cycle of the apparent heat flow modulation, which is of ≈15-km wavelength and ≈200-mW/m2 amplitude. This variation appears to be uncorrelated with the topographic variation of ≈10-km wavelength. Assuming that the porosity sensitive electromagnetic results of Young and Cox (1981) reflect a permeability distribution that restricts the depth of circulation to ≈1.4 km, average temperatures of circulation may be ≈50-60 ¿C. Deeper, higher-temperature circulation is allowable if a more uniform permeability-depth distribution holds.