After ageing in water for 4 to 5 h, the permeability of excised squash hypocotyl sections to urea, as measured in hypotonic solutions, had declined to about 50% of freshly excised tissues. This degree of change was common to several members of the cucurbit family. The permeability change was relatively temperature insensitive and was not closely linked to segment length. It was unaffected by puromycin, cycloheximide, and actinomycin D. Adenosine phosphate compounds, indole-3-acetic acid, sulfhydryl reagents, and 0.5 mMCaSO4were also ineffective in altering the course of permeability change. However, the exposure of sections to osmotica (mannitol, glycerol, KCl) during ageing, at concentrations close to those of solutes found in the apparent free space of squash hypocotyls, significantly inhibited the decline of urea permeability. Once the decline in hypotonic solutions was expressed, it was not reversed by incubation in isotonic solutions in contrast with the short-term effects of osmotica on permeability in freshly excised sections. Apparently, the ageing-dependent decline in urea permeability by squash hypocotyl cells is the expression of a long-term osmotic regulatory process (irreversible) which complements the functioning of a short-term osmotic control system (reversible).