Foliar NH4(+) exposure is linked to inhibition of lateral root (LR) formation. Here, the role of shoot ethylene in NH4(+)-induced inhibition of LR formation in Arabidopsis was investigated using wild-type and mutant lines that show either blocked ethylene signalling (etr1) or enhanced ethylene synthesis (eto1, xbat32). NH4(+) exposure of wild-type Arabidopsis led to pronounced inhibition of LR production chiefly in the distal root, and triggered ethylene evolution and enhanced activity of the ethylene reporter EBS:GUS in the shoot. It is shown that shoot contact with NH4(+) is necessary to stimulate shoot ethylene evolution. The ethylene antagonists Ag(+) and aminoethoxyvinylglycine (AVG) mitigated LR inhibition under NH4(+) treatment. The decrease in LR production was significantly greater for eto1-1 and xbat32 and significantly less for etr1-3. Enhanced shoot ethylene synthesis/signalling blocked recovery of LR production when auxin was applied in the presence of NH4(+) and negatively impacted shoot AUX1 expression. The findings highlight the important role of shoot ethylene evolution in NH4(+)-mediated inhibition of LR formation.