GH synthesis has been documented in canine mammary tissue and mammary tumors. In the present report, the characteristics of the GH receptor (GHR) are studied in these tissues. First, using immunohistochemistry, GHR was found to be present throughout normal and tumorous mammary tissues, being localized in epithelial and myoepithelial/spindle cell components and in the activated fibroblasts of desmoplastic tumor stroma. GHR expression seemed to be down-regulated only in terminally differentiated alveoli in normal tissue. GHR immunoreactivity in particular mammary (adeno)carcinomas was heterogenous. Second, the canine GHR was characterized at the molecular level. Northern blot analysis revealed a major GHR transcript of approximately 4.2 kb. The coding sequence of the canine GHR shows extensive homology with the GHR of several species. Seminested RT-PCR (using primers annealing in exons 4-5, exon 6, and exon 9) generated, next to the primary product, four different products in mammary tissues and the canine mammary tumor cell line CMT-U335, which seemed to be alternative GHR transcripts. These alternative GHR transcripts were characterized by exon 8 skipping, exon 7 skipping, and use of alternative splice donor and acceptor sites. Especially, the transcript that is missing exon 8 may encode a GH binding protein. In most malignant mammary samples, only the primary transcript was present; and alternative transcripts could not be detected. The absence of alternative GHR transcripts in mammary carcinomas, and thus putative inhibitors of GH-induced signal transduction, may contribute to enhanced sensitivity of malignant tumors to GH.