The putative role of cellular or solvent volume in protrusive activity and locomotion has been investigated in blebbing Walker carcinosarcoma cells using hypertonic media. Blebbing, locomotion, and cell polarity are completely suppressed by 0.2 M sorbitol. The response occurs in two steps. In a first step, i.e. within 10 sec after the addition of sorbitol, blebbing and locomotion are inhibited and this is associated with an average cell volume reduction by 17% (corresponding to a reduction in solvent volume by 38%). It clearly precedes suppression of cell polarity (pre-existing protrusions, tail) occurring in a second step within 5 to 10 min after addition of sorbitol without additional reduction in the cell or solvent volume. The relative amount of F-actin does not correlate with the decrease in cell volume, suppression of blebbing, locomotion, and cell polarity. A significant decrease in the relative amount of F-actin is found only at volume reductions which are higher than those required to completely suppress blebbing, locomotion, and cell polarity. F-actin staining occurs preferentially along the cell membrane in isotonic as well as in hypertonic media. The results are best compatible with the hypothesis that hydrostatic pressure rather than actin polymerization at the front is the direct force driving the membrane forward during bleb formation. Cells with lamellipodia show a similar response to hypertonic media, suggesting that basically similar mechanisms may operate in both forms of protrusions.