Low Km GTPase activity was studied in the purified membranes from the Lymnaea stagnalis CNS. Stimulation of GTPase activity of G proteins by dopamine in membrane preparations is transient; it is followed by reduction of the activity. Magnesium ions regulate activity of G proteins; thus, the value of [Mg2+]i is an important factor to be taken into account in investigations of functioning of G proteins. In order to determine the mean physiological level of the neuronal [Mg2+]i, neuronal elements of the molluscan CNS were separated from other cell types. The values of 1.16±0.19 and 1.49±0.08 mM were obtained for [Mg2+]i of the neuronal cells with Mag-fura-2 and Mag-fura-red, respectively. An inhibitory effect of dopamine was displayed within the range of free Mg2+ concentrations from 3 μM to 3 mM. GTPase activity was stimulated with Na+ in a dose-dependent manner. However, GTPase activity became independent of Na+ in the presence of dopamine. Replacing of ATP by 5′-adenylyl imidodiphosphate suppressed the inhibitory effect of dopamine, but did not completely prevent the dopamine influence. The effect of dopamine was dose-dependent and did not decrease after PTX treatment. The inhibitory influence of dopamine on the GTPase activity was reversed by a selective antagonist of the dopamine D2 receptors, S(-)-sulpiride.