Cystic fibrosis (CF) is caused by the malfunctioning of CFTR, an anion channel that either directly or indirectly mediates Cl^–, and HCO₃^–-mediated fluid secretion in many exocrine organs, including the submucosal glands of the airways. People with CF have defective lung defenses, and so develop chronic lung infections with bacteria and fungi that are localized in static mucus within the airways (for review see Wine & Joo, 2004). Because glands produce most airway mucus, we developed optical methods to quantify secretion by individual glands (Joo et al. 2001), and discovered that the glands of people with CF no longer secrete in response to VIP or forskolin (Joo et al. 2002). The glands also show more subtle defects to cholinergic stimulation (Jayaraman et al. 2001). To better understand how glands elaborate mucus, we initiated studies of gland secretion using differential interference contrast (Nomarski or DIC) microscopy. These studies can potentially localize the cellular origins of various kinds of secretion and can give a host of new insights in gland function. As one example, immunohistochemistry has localized CFTR in two different regions of the glands: either primarily in the serous cells (Engelhardt et al. 1992) or primarily in the ciliated ducts (Kreda et al. 2005). To test the hypothesis that the ducts, rather than the acini, might be the origin of most CFTR-mediated fluid secretion, we used Nomarski DIC microscopy to track the flow rates of particles within the mucus of forskolin-stimulated glands. Within the ducts, particles flowed at a constant rate, indicating that neither secretion nor absorption was occurring, but in the acini they accelerated, indicating secretion. This was confirmed by micro-ligation experiments of isolated glands. When the junction between the unbranched duct and the rest of the gland was ligated and the gland was stimulated, no mucus exited the duct orifice, but the acinar region swelled. With double ligations that isolated the ducts, the acinar region swelled after stimulation but the duct neither swelled nor contracted. These results support the functional location of CFTR in acinar serous cells, but they leave open the function of CFTR in the duct. One possibility is that CFTR (and ENaC) in the ducts are primarily absorptive, and then in cystic fibrosis glands, hyperabsorption via unregulated, overactive ENaC might alter mucus in the duct so that its secretion is blocked. To test that we applied ENaC blockers to normal and CF glands, but these did not alter the volume of gland secretion, supporting the proposal that hyposecretion, not hyperabsorption, is the basic defect of CF glands (Joo et al. 2006).