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The effect of AlCl₃ on the (Na⁺/K⁺)ATPase activity of freeze–thawed synaptosomes, isolated from rat brain cortex, has been studied. The AlCl₃ action on the enzyme hydrolytic activity was examined using in vitro and in vivo approaches. Following exposure to AlCl₃ using both in vitro (synaptosomes incubated in the presence of AlCl₃ for 5 min) and in vivo (synaptosomes isolated from rats that received 0.03 g AlCl₃/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 AlCl₃ concentration >75 μM and at non-limiting ATP concentrations. Conversely, AlCl₃ 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 AlCl₃-induced inhibitory …