An experimentally-based modelling technique was applied to describe quantitatively the uptake, translocation, storage, and assimilation of NO3- and H2PO4- over a 9d period in mid-vegetative growth of sand-cultured castor bean (Ricinus communis L.) which was fed 12 mM NO3- and either 0.5 or a severely limiting 0.005 mM H2PO4-. Model calculations were based on increments or losses of NO3- and reduced N or of H2PO4- and organic P in plant parts over the study period, on the concentrations of the above compounds in xylem and phloem sap, and on the previously determined flows of C and N in the same plants (Jeschke et al., 1996).Modelling allowed quantitative assessments of distribution of NO3- reduction and H2PO4- assimilation within the plant. In control plants 58% of total NO3- reduction occurred in leaf laminae, 40% in the root and 2% in stem and apical tissues. Averaged over all leaves more than half of the amino acids synthesized in laminae were exported via phloem, while the root provided 2.5-fold more amino acids than required for root growth. P deficiency led to severe inhibition of NO3- uptake and transport in xylem and even greater depression of NO3- reduction in the root but not in the shoot. Accentuated downward phloem translocation of amino acids favoured root growth and some cycling of N back to the shoot.In control plants H2PO4- was the principal form of P transported in xylem with young laminae acting as major sinks. At the stem base retranslocation of P in the phloem amounted to 30% of xylem transport. H2PO4- assimilation was more evenly distributed than NO3- reduction with 54% occurring in leaf laminae, 6% in the apical bud, 19% in stem tissues, 20% in the root; young tissues were more active than mature ones. In P-deficient plants H2PO4- uptake was severely decreased to 1.8% of the control. Young laminae were the major sink for H2PO4-. Considerable remobilization of P from older leaves led to substantial shoot to root translocation via phloem (50% of xylem transport). Young leaf laminae were major sites of H2PO4- assimilation (50%), followed by roots (26%) and the apical bud (10%). The remaining H2PO4- was assimilated in stem and mature leaf tissues. Old leaves exhibited 'negative' net assimilation of H2PO4-, i.e. hydrolysis of organic P exceeded phosphorylation. In young laminae of low P plants, however, rates of H2PO4- assimilation per unit fresh weight were comparable to those of the controls.