The distribution of nutrient flux (geostrophic velocity times concentration) in five sections across the Gulf Stream--North Atlantic Current (from the Florida Straits to 35¿W) is characterized by an intense core, centered at the depth of the 26.8 &sgr;t isopycnal surface (typically 500 m). This ''nutrient stream'' carries nutrient transport of the order of 103 kmol s-1 of nitrate and proportional amounts of other nutrients. Between the Florida Straits and the Mid-Atlantic Bight, water transport doubles, but nutrient transport trebles, because along-isopycnal inflow from the subtropical gyre is concentrated in the layers of the upper thermocline, which are rich in nutrients. Beyond the Mid-Atlantic Bight, both water and nutrient transports decline slowly. Water mass and nutrient balances of nine isopycnal layers reveal significant upward entrainment and mixing of thermocline waters in the sector of the stream between the Florida Straits and the Mid-Atlantic Bight. A two-box model of the nutrient-depleted surface layers (&sgr;t<26.8) and the nutrient-rich thermocline layers (26.8<&sgr;t<27.5) shows an upward entrainment rate of about 1.6 m2 s-1 per unit length of the stream, or a diapycnal velocity of 2¿10-5 m s-1 over the 80-km width of the stream. In addition, there is two-way diapycnal mass exchange at approximately the same rate. The rate of inflow from the surface layers of the Sargasso Sea is about 12¿106 m3 s-1, from the thermocline layers 15¿103 s-1. ¿ American Geophysical Union 1991