Effects of varied nutrition on the inheritance mode of the total and specific length of roots, root weight, shoot : root ratio and root uptake efficiency were studied in a diallel set of spring barley. Plants were grown in sand-vermiculite cultures under high and low NP nutrition. Results of the analysis of variance indicated that both the general (GCA) and specific (SCA) combining ability effects were important for the total length and dry matter of roots formed at the vegetative growth stage. Preponderance of non-additive gene action and overdominance were apparent only for root dry matter. The specific root length and root uptake efficiencies were controlled by additive gene action. Genotype ? nutrition interactions were significant for the root length measures and uptake efficiency indices. Although significant GCA ? nutrition interaction effects were observed for these root characters, the interactions contributed mostly to the magnitude of GCA effects but not to their sign. Except for root dry matter, the contribution of non-additive gene action to root variation enhanced under stress, heritabilities declined, while degrees of dominance of genes tended to increase. Nevertheless, the moderate narrow-sense heritabilities (18-62%) and the considerable proportion of additive variance found for the root characters under nutrient shortage suggest that an improvement of rooting ability under less favourable nutrition through conventional selection is a real objective for barley breeders.