Experiments have been done to test the idea that mucins undergo postexocytotic swelling. Previous work led to the hypothesis that the glycoprotein network of the mucus gel is probably held together by entanglements and low energy bonds, rather than by interchain covalent bonding. Since glycoproteins and other proteins in the mucus are polyions. it was further proposed that mucus must be capable of swelling, with its swelling properties depending on the pH and ionic strength of the medium hydrating the mucus. Experiments using oestrous cervical mucus from cows as a model confirmed this prediction. Observations on tissue cultures of respiratory secretory cells reported here show that freshly secreted mucins also undergo swelling during and after exocytosis. This evidence supports the hypothesis that the rheological properties of mucus may be physiologically regulated by hydration via control of the transepithelial movement of water. ions and soluble proteins. rather than by variations in the degree of covalent cross-linking between glycoprotein chains as proposed earlier.