Hydro stations were worked during August/September, 1968, over six basins in the floor of the Median Valley of the Mid-Atlantic Ridge near latitude 45 °N, and in the open Atlantic each side of the Ridge. Measurements were made of temperature, salinity, dissolved oxygen, reactive silica, and dissolved fluoride.Water properties in the valley are practically homogeneous below a depth of about 2390 m, and are controlled by the depths of saddles in the adjacent ridge systems. No thermal or chemical anomalies were found in waters within the basins which might have been due to geothermal action in the valley floor. Relatively high concentrations of dissolved oxygen in basin waters suggest that the valley is efficiently flushed.