Smooth muscle cells (SMC) of the human urinary bladder are believed to be electrically coupled. However, it has not been successfully demonstrated the nature of the underlying cell-cell communication. Here, we used Western blot technique to demonstrate the gap junction protein connexin 43 (Cx 43) in human bladder musculature and in smooth muscle cell cultures. We found expression of Cx 43 in all samples of the detrusor (Mdv, n=6) and in five of six samples of the internal sphincter (Mv). Cx 43 expression was less in cell cultures, 60% showing Cx 43 expression. Iontophoretic application of the gap junction permeable fluorescent dye lucifer yellow revealed a mean of 2.6±2.2 (mean±SD, n=8) coupled cells in cultured smooth muscle cells. Our findings support the notion that Cx 43 is constitutively expressed in human bladder SMC in vivo. Thus, the function of the bladder is likely to be influenced by the organisation of smooth muscle cells into functional syncytia.