The cytochrome P-450 2C18 gene was found by reverse transcription-PCR to represent the most abundantly expressed gene of the P-450 2C subfamily in human epidermis. However, in addition to the canonical mRNA of nine exons, transcripts that have skipped exon 4 or 5, exons 4, 5, and 6, or exons 4, 5, 6, and 7 were also identified in this tissue. Remarkably, circular RNA transcripts synthesized by the joining of the donor and acceptor splice sites of the same exon were detected in human epidermis for exons 4 and 5. Moreover, molecules composed of exons 4, 5, and 6 with the donor splice site of exon 6 joined to the acceptor splice site of exon 4 or composed of exons 4, 5, 6, and 7 with the donor splice site of exon 7 joined to the acceptor splice site of exon 4 were also found to be present in this tissue. In rat testis, a similar analysis allowed the detection of a circular RNA molecule composed of exons 6 and 7 of the androgen binding protein (ABP) gene, with the donor splice site of exon 7 joined to the acceptor splice site of exon 6, and of an ABP mRNA which had skipped exons 6 and 7. These results apparently substantiate the hypothesis that alternative pre-mRNA splicing has the potential to generate not only mRNAs that lack one or more exons but also circular RNA molecules that are composed of the exons that are skipped. However, additional 2C18 circular species containing various combinations of exons were also detected in human epidermis, and an exon 6-skipped ABP mRNA molecule was identified in rat testis. This observation is interpreted as indicative that at low frequency, numerous circular RNA formation and exon skipping events may occur, allowing the joining of a variety of different combinations of exons. Moreover, the relative stability of these molecules is apparently the key factor that determines the relative ease of their detection.