The molecular basis of water-permeability in salivary and other exocrine glands is not understood. We have examined two well-studied salivary epithelial cell lines for evidence of a Hg-inhibitable water-permeability pathway. A5 and HSG cells are derived from rat and human submandibular glands, respectively. Only A5 cells demonstrated such a pathway. The rate of A5 cell osmotic shrinkage was inhibited about fivefold in the presence of 300 microM HgCl2. To determine if this activity was associated with the expression of the prototypical water channel (aquaporin, AQP) AQP1, we used three separate experimental approaches; Northern analysis and reverse transcription-polymerase chain reaction (RT-PCR) analysis of isolated mRNA, and Western analysis of cell membranes. All three methods yielded positive results with A5 cells and negative results with HSG cells. The approximately 800 bp product of RT-PCR was analyzed further by sequencing and restriction enzyme digestion. The results were consistent with the previously reported coding region sequence for rat kidney AQP1. The aggregate data demonstrate that marked differences in water-permeability and water channel expression exist in these two salivary epithelial cell lines.