Fusaric acid, at a concentration of 1mM induced at pH 5.5 an early hyperpolarisation was followed by a marked depolarisation of membrane potential difference. During this time increased electrolyte leakage from the primary Ricinus roots was determined. At higher pH values (6.5 and 8) the depolarisation caused by fusaric acid was immediate without hyperpolarisation observed at pH 5.5. Simultaneous exposure of the roots to P-ATPase activator fusicoccin and fusaric acid (pH 6.5) diminished the hyperpolarising effect of fusicoccin. The present results suggest that the dissociated form of fusaric acid does directly affect particular cell targets (plasmalemma, mitochondria) and viability of root cells decreased with the time of exposure and concentration of fusaric acid.