The mouse κ opioid receptor (KOR) gene uses two functional polyadenylation signals, separated by a distance of approximately 2.2 kilobases (kb) in the 3′-end of the gene. As a result, two major groups of KOR transcripts, with sizes of approximately 1.6 and 3.8 kb, respectively, are detected in mouse tissues and P19 cells. Utilization of different poly(A) of the KOR gene produces KOR transcripts of different mRNA stability, transcription efficiency, and regulatability. Retinoic acid specifically suppresses the expression of KOR transcripts using the second poly(A) in P19 cells. A putative transcriptional enhancer region is present within the second 3′-untranslated region (3′-UTR). It is concluded that alternative polyadenylation of the mouse KOR transcripts results in differential regulation of KOR expression at both transcriptional and post-transcriptional levels. A negative regulatory pathway for KOR transcription involves a putative enhancer region in its 3′-UTR. KOR mRNAs using the second poly(A) is more stable than that using the first poly(A).