The rate of liquid flow per unit area (Jv/A) through Madin-Darby canine kidney cell monolayers has been studied at temperatures between 0 and 38 degrees C and at transmonolayer hydrostatic pressures between 14 and 44 cmH2O. Jv/A decreased exponentially with time during application of a constant pressure to the free surface of each monolayer. This behaviour resembles the sealing of cultured vascular endothelium. For monolayers sealed between 33-38 degrees C and 30-33 cmH2O, the mean (+/- S.E.M.) half-time (t1/2) of sealing was 228 (+/- 88) s (n = 6). The decrease in Jv/A during sealing can be expressed as a fraction of the initial Jv/A. Between 33-38 degrees C and 30-33 cmH2O, the mean (+/- S.E.M.) sealing fraction was 0.58 (+/- 0.06; n = 6). Mean sealing t1/2 was longer at lower temperatures, and longer for glutaraldehyde-fixed than for unfixed monolayers, but did not vary with transmonolayer pressure. Sealing fraction was not affected by variations in temperature or transmonolayer pressure, or by glutaraldehyde fixation of monolayers. It is argued that sealing is a physical, rather than a biological, phenomenon and that monolayers have non-linear mechanical properties.