The mechanism involved in the transport of assimilates through the stalk by which the grain is attached to the rachilla has been investigated. Based on the presumption of pressure-driven mass flow, treatments designed to inhibit the import of assimilates would be expected to inhibit also the entry of water into the grain. Detached ears were supplied with solutions of [¹⁴C]sucrose in tritiated water (THO), and ears attached to the plant were watered with THO and supplied with ¹⁴CO₂. The rachilla was heated by contact with a hot wire, and the stalk by steam-girdling. In addition, dinitrophenol (DNP) was used as an inhibitor of metabolically linked transport mechanisms. Heating or treatment with DNP reduced the transport of [¹⁴C]sucrose or [¹⁴C]-labelled assimilate into the grain, but the entry of THO was not reduced even by treatments which abolished altogether the transport of ¹⁴C into the grain. These results cannot be reconciled with mass flow as the means of transporting assimilates through the stalk of the wheat grain during grain filling.