Urea-stimulated volume reduction in SS reticulocytes is irreversible. Cells were treated with nystatin and incubated in HBS pH 7.4 plus 600 mM urea. The solid curve represents volume reduction in cells continuously exposed for 120 minutes. At 10, 20, and 60 minutes, samples were removed, centrifuged, and resuspended in HBS pH 7.4 without urea and incubated at 37°C for the duration of the 120-periond (open symbols, connected by arrows to the points representing their exposure time to urea). Data are means of 3 independent experiments, with error bars representing SEM.