In plants of Sinapis alba induced to flower by exposure to a single long day (LD), previous work demonstrated that the movement of a shoot-to-root signal early during the photoextension period of the LD was essential for flowering. Interrupting this movement by bark ringing (girdling) of the stem inhibited the floral response of plants to the LD. In the present work we show that (a) the girdling treatment decreases the soluble sugar level in the roots of induced plants, and (b) the inhibitory effect on the floral response caused by girdling can be completely overcome by supplying the roots with sucrose directly at appropriate times. Thus, we demonstrate that sucrose moving in the phloem is the shoot-to-root signal essential for flowering. We have also found that one of the major effects of the extra-sucrose on the roots is to stimulate the root-to-shoot movement of [9R]Z, the predominant cytokinin of the xylem sap in Sinapis. The importance of this upward movement of [9R]Z for flowering is indicated by the observation that (a) the floral response to the LD is inhibited by growing plants in an atmosphere saturated with water (impairing upward movement of xylem sap) during the LD itself, and (b) the inhibitory effect on the floral response caused by girdling, which markedly reduces [9R]Z export from roots, is relieved by direct application of BA, a cytokinin, to the apex. Other possible effects of the shoot-derived sucrose on roots in relation to flowering are also discussed. Our results show that a shoot-to-root-to-shoot physiological loop is essential for flowering in intact Sinapis plants.