The effect of AlCl 3 on the (Na +/K +)ATPase activity of freeze–thawed synaptosomes, isolated from rat brain cortex, has been studied. The AlCl 3 action on the enzyme hydrolytic activity was examined using in vitro and in vivo approaches. Following exposure to AlCl 3 using both in vitro (synaptosomes incubated in the presence of AlCl 3 for 5 min) and in vivo (synaptosomes isolated from rats that received 0.03 g AlCl 3/day for 4 months) approaches, the (Na +/K +)ATPase activity was inhibited in a concentration-dependent way. The maximal inhibitory effect (∼60%) was observed in the presence of a AlCl 3 concentration >75 μM and at non-limiting ATP concentrations. Conversely, AlCl 3 did not inhibit the enzyme activity when UTP was used as substrate instead of ATP. Analysis of the substrate dependence of membrane-bound (Na +/K +)ATPase by a computer simulation model suggests that the AlCl 3-induced inhibitory effect is characterised by a reduction of the rate-limiting step velocity of the reaction cycle. Moreover, it seems that aluminium can induce impairment of the interprotomeric interaction within the oligomeric ensemble of membrane-bound (Na +/K +)ATPase. In fact, this effect was accompanied by a slight, but significant, decrease of readily accessible SH groups, which are involved in the maintenance of the membrane-bound (Na +/K +)ATPase oligomeric structure. In conclusion, during exposure to aluminium, reduction of the activation of membrane-bound (Na +/K +)ATPase by high ATP concentrations occurs, which results in a partial inhibition of the enzyme.