Mucus coating epithelia is one of the most critical components of innate defense against injury and infection of internal organs. To become protective, however, mucins, which are the longest, largest molecules in biology, must unravel from intensely condensed structures inside intracellular granules to occupy volumes 3 orders of magnitude larger as extracellular molecular ‘mucus’. Impressively, the transformation occurs within seconds at most. Having found that the formation of “mucus’ is critically dependent on the presence of HCO3- (1) (2), we now address the properties of mucin / HCO3- interactions and secretions. We surmise that HCO3- effects mucus unraveling largely by competing away Ca2+ that divalently bridges the innumerable anionic sites fixed to the mucin peptide polymers, but other binding may also be involved. Using pH stat and transepithelial electrical measurements of HCO3- flux, we show that HCO3- is secreted in the intestines, airways, and reproductive tracts. Using histochemical staining of mucus (3) with morphometry, we determine relative losses of mucin from goblet cells. By applying selective inhibitors (GlyH-101, DIDS, acetazolamide) and agonists (Isoproterenol, Forskolin/IBMX, PGE2, Carbachol, UTP, VIP) in the presence and absence of HCO3-, we illustrate the impact of HCO3- secretion on mucin exocytosis and mucus discharge in these systems. These results indicate that secretion of gel forming mucins and HCO3- secretion are regulated independently and in separate cells. It appears that exocytosis of gel forming mucins from goblet cells is largely under Ca2+ mediated control, while HCO3- secretion derives from surrounding secretory cells is largely under cAMP mediated control of CFTR. Thus, even though HCO3- may affect the exocytosis of mucins, the luminal secretion of HCO3- appears to be required to support normal mucus maturation and formation. The relevance of these results to pathogenesis will be considered.