The aim of the present study was the characterization of the ovine aromatase cytochrome P450 encoding gene (Cyp19) and the analysis of its tissue-specific expression. Two loci with considerable sequence identity were found (Cyp19 and Cyp19b). From Cyp19, tissue-specific transcript variants with different untranslated first exons but identical coding regions could be identified. Cyp19b transcripts were not detected. In the sheep brain and ovarian granulosa cells transcript variants, starting with the untranslated exons 1.4 and 2, respectively, were preferentially found. Exons 1.2 and 1.3 which had been described in bovines could not be detected in sheep and the major 5' untranslated region of the bovine placental transcript, exon 1.1, was also not found to predominate in the sheep placenta. However this exon frequently was combined with a new untranslated exon (exon 1.1a) thus generating an alternative splice variant. The main placental transcripts in sheep had a different first exon (exon 1.5). Two alternatively spliced variants of this transcript were found with tissue-specific preference. From the present data it can be concluded: (a) that the ovine genome contains two copies of Cyp19 of which only one is transcribed and may encode a functional protein; and (b) that in spite of being closely related species, sheep and cattle have remarkable differences concerning tissue-specific transcript distribution and presumable promoter usage.