In this study, primary roots sprouting from Lycopersicon esculentum Mill. (tomato) seeds, germinated in Hoagland's nutrient solution containing different amounts of ZnCl(2) (1, 3 and 5 mM), were observed. Primary roots have shown negative geotropic behaviour at ZnCl(2) levels of 3 and 5 mM, instead of positive tropism. This effect was much obvious in the plants grown in Hoagland solution containing 5 mM ZnCl(2). On the other hand, the addition of 100 mg/L triiodobenzoic acid (TIBA) into imbibition and growth medium containing 3 or 5 mM ZnCl(2) eliminated negative gravitropic behaviour of primary roots and induced positive tropism of roots which were oriented to embryo axis, and gave rise to root elongation.