In order to probe the interaction between the neurotransmitter receptors and guanine nucleotide-binding regulatory (G) proteins in rat hippocampus, the high-affinity GTPase activity stimulated by 5-hydroxytryptamine (5-HT), gamma-aminobutyric acid (GABA), carbachol (CCh), and dopamine (DA) has been investigated, focusing on the additivity among the effects of these agonists at their maximally effective concentrations. There were simple additive relationships among 5-HT-, GABA-, and CCh-stimulated activities. As 5-HT, GABA-, and CCh-stimulated high-affinity GTPase activities are mediated by the 5-HT1A, GABAB, and pirenzepine-insensitive muscarinic receptors, respectively, the additive effects indicate that these three receptors are independently coupled to distinct pools of G proteins. In contrast, an apparent lack of additivity was seen between 5-HT- and DA-stimulated activities. This phenomenon was likely due to an activation of the common 5-HT1A receptor-mediated signalling by DA as well as 5-HT, since the DA-sensitive increment of the activity was potently inhibited by the 5-HT1 receptor antagonist methiothepin, but not by the DA D2 receptor antagonist raclopride.