The formation of blisters is a transient phenomenon that led Dr. Leighton to describe its observation with time-lapse photography in the MDCK monolayer to a "gently boiling oatmeal." One may ask why is it transient and why most areas are momentarily not blistering. The observations discussed in this article indicate that (a) at the blister, junctions are really tight; (b) when transported fluid is allowed to escape through a permeable support, junctions are also tight, but (c) when the support is impermeable the junctions allow ouabain and peroxidase through. This suggests that, if the attachment of the monolayer is strong enough, the accumulation of fluid (Figure 7) bursts the junctions. If, on the contrary, junctions withstand the pressure, factor 3 of Figure 7 prevails over the others, and a blister is formed. In all the rest of the monolayer junctions seem to be open. This is in keeping with the observation by Rabito et al.,27 that if the monolayers are prepared on weakened supports, blisters are much bigger than under control conditions. It also allows the measurement of ionic fluxes and labeling of pumping sites at the basolateral membrane. As a corollary, one may say that some of the factors known to affect blistering 26 may very well act through modifications in the occluding junctions.