Canine mammary gland has been identified as a major site of the extrapituitary growth hormone (GH) production. This finding is linked to its role in tumourigenesis of the mammary gland. Our previous studies indicated the role of GH and GH receptor (GHR) in regulation of proliferation and apoptosis. Thus, we have optimized the ghr RNA interference method in canine mammary carcinoma cell line CMT-U27. We have analysed the effect of GHR reduction on the intracellular signalling and the cell cycle and apoptosis. The results showed that GHR reduction decreased the p-ERK1/2 expression and caused increase of apoptosis and decrease in number of cells at S and G2M phases. This study indicates that GHR besides proliferative effect promotes growth by increasing cell survival. It can tilt the balance between proliferation and death in cancer cells.