In the bovine mammary gland, extracellular matrix proteins such as collagen I and IV and laminin coexist around the alveoli during lactation. In contrast, connexin, a gap junction protein, was scarce in lactating mammary epithelial cells. In addition, collagen IV and laminin were shown to be involved in alpha-casein synthesis and secretion in the bovine mammary epithelial cells. The connexin proteins, which were detected during the non-pregnant and dry periods, were scarce at peak lactation. It was shown that extracellular matrix proteins are important for the stimulation of casein synthesis, but this was accompanied by a suppression of connexin expression.