Flows of abscisic acid (ABA) were investigated in whole plants of castor bean (Ricinus communis) grown in sand culture under either phosphate deficiency or moderate salinity. Xylem transport of ABA in P-deficient plants was stimulated by a factor of 6 whereas phloem transport was affected only very slightly. ABA deposition into leaves of P-deficient plants was not appreciably different from the controls because of strong net degradation in leaves. Since conjugation of ABA was strongly reduced in all organs of P-deficient plants ABA was presumably metabolized mainly to phaseic acid and dihydrophaseic acid. The increased import of ABA occurred predominantly into fully differentiated but not senescent leaves and showed a good correlation with the inhibition of leaf conductance under P deficiency.