Subtype diversity of heterotrimeric G proteins and G protein-coupled receptors enables a wide spectrum of signal transduction. However, the significance of isoforms within receptor or G protein subfamilies has not been fully elucidated. In the present study, we have tested whether α2-adrenoceptors require specific Gα isoforms for their function in vivo. In particular, we analyzed the role of the highly homologous Gαi isoforms, Gαi1, Gαi2, and Gαi3, in typical α2-adrenoceptor-controlled functions. Mice with targeted deletions in the genes encoding Gαi1, Gαi2, or Gαi3 were used to test the effects of α2-adrenoceptor stimulation by the agonist medetomidine. The α2-adrenoceptor agonist medetomidine inhibited [3H]norepinephrine release from isolated prefrontal brain cortex or cardiac atria tissue specimens with similar potency and efficacy in tissues from wild-type or Gαi-deficient mice. In vivo, bradycardia, hypotension, induction of sleep, antinociception, and hypothermia induced by α2-adrenoceptor activation did not differ between wild-type and Gαi-knockout mice. However, the effects of the α2-agonists medetomidine or 5-bromo-6-(2-imidazolin-2-ylamino)quin-oxaline tartrate (UK14,304) on spontaneous locomotor activity or anesthetic sparing were reduced or absent, respectively, in mice lacking Gαi2. In microdissected locus coeruleus neurons or postganglionic sympathetic neurons from stellate ganglia, all three Gαi subunits were expressed as determined by quantitative reverse transcription-polymerase chain reaction, with Gαi1 and Gαi2 dominating over Gαi3. Functional redundancy of the highly homologous Gαi isoforms may predominate over specificity to regulate distinct intracellular pathways downstream of α2-adrenoceptors in vivo. In contrast, inhibition of locomotor activity and anesthetic sparing may be elicited by a specific coupling of α2A-adrenoceptors via the Gαi2 isoform to intracellular pathways.