We investigated the cross-talk between auxin (IAA) and ethylene in the control of root growth of Arabidopsis plants (Arabidopsis thaliana). The root growth of ethylene insensitive etr1-1 and auxin insensitive tir1 mutants under the effect of IAA and 1-methylcyclopropene (1-MCP, inhibitor of ethylene perception) was compared, respectively, with that of the wild type ecotype Columbia (Col-0). Roots of Col-0 were shorter than those of the etr1-1 mutant. The addition of IAA (5.7 × 10 ^–6 М) to the growth medium led to 20 % root shortening in Col-0 plants, but not in etr1-1 mutants. Thus, the capacity of plants for ethylene perception contributes to the control of root length and its sensitivity to IAA. Roots of etr1-1 were less heavy than in Col-0, indicating that ethylene maintains root mass accumulation. Treatment with IAA caused a decrease in root mass of both genotypes (resulting in a 25 and 10 % decline in the root mass of Col-0 and etr1-1 as compared to the corresponding control), suggesting that IAA may influence root biomass accumulation independently of ethylene. However, sensitivity to ethylene increases plant responsiveness to IAA. Mutation in the auxin receptor decreased the sensitivity of roots to inactivation of ethylene receptors: treatment with 1-MCP resulted in a 40 % decline in the root mass of Col-0 and only a 10 % decrease in tir1. The decrement in sensitivity to auxins in the tir1 mutant decreased the responsiveness of root biomass to 1-MCP-treatment. These data suggest an additive action of ethylene and auxins on accumulation of root biomass.