A partial cDNA encoding most of the third intracellular loop of the mouse α_1d‐adrenergic receptor subtype was amplified from hippocampus by reverse transcription‐polymerase chain reaction (RT‐PCR) using degenerate oligodeoxynucleotide primers. This DNA fragment was used as a probe to isolate an α_1d‐adrenergic receptor cDNA from a mouse brain cDNA library. The deduced amino acid sequence encodes a potential protein of 562 amino acids, and northern hybridization of poly(A)⁺ RNA isolated from mouse brain detected a single 3.0‐kb transcript. Partial cDNA fragments of the α_1b‐ and α_1a‐adrenergic receptor subtypes were also amplified from mouse brain and sequenced. Analysis of the mRNA expression by RT‐PCR indicated that the α₁‐adrenergic receptors are widely distributed in mouse tissues. The α_1d subtype is expressed in brain areas such as hippocampus, striatum, and brainstem and also in many extracerebral tissues, such as lung, liver, heart, kidney, and spleen. The α_1a subtype is also expressed in many tissues, whereas the α_1b subtype has a more restricted expression, with high levels in striatum, brainstem, and diencephalus.