PURPOSE: Circular RNAs (circRNAs), a novel class of RNAs, perform important functions in biological processes. However, the role of circRNAs in the mammary gland remains unknown. The present study is aimed at identifying and characterizing the circRNAs expressed in the mammary gland of lactating rats. METHODS: Deep sequencing of RNase R-enriched rat lactating mammary gland samples was performed and circRNAs were predicted using a previously reported computational pipeline. Gene ontology terms of circRNA-producing genes were also analyzed. RESULTS: A total of 6,824 and 4,523 circRNAs were identified from rat mammary glands at two different lactation stages. Numerous circRNAs were specifically expressed at different lactation stages, and only 1,314 circRNAs were detected at both lactation stages. The majority of the candidate circRNAs map to noncoding intronic and intergenic regions. The results demonstrate a circular preference or specificity of some genes. DAVID analysis revealed an enrichment of protein kinases and related proteins among the set of genes encoding circRNAs. Interestingly, four protein-coding genes (Rev3l, IGSF11, MAML2, and LPP) that also transcribe high levels of circRNAs have been reported to be involved in cancer. CONCLUSION: Our findings provide the basis for comparison between breast cancer profiles and for selecting representative circRNA candidates for future functional characterization in breast development and breast cancer.