MICROVILLI have various functions connected with an increase in surface area. In the form of brush border of intestinal and renal epithelium, microvilli increase the absorptive capacity of cells. On tumour cells1,2 they may have a role in invasiveness3. Their presence on cultured cells4, which is apparently controlled by the intracellular concentration of cyclic AMP5, increases the surface area so as to facilitate “spreading”6 and cytokinesis7,8 without concomitant membrane synthesis. In the latter two situations, cell volume remains constant, while the shape changes. The most common situation in which the reverse occurs, that is an increase in surface area accompanying an increase in volume without a change in shape, is during cell swelling. Swelling due to entry of water is caused by (1)exposure of cells to a medium hypotonic relative to that of the cell content or (2) an increase in passive diffusion of cations9, such that the Na/K pump cannot maintain an asymmetric intracellular/extracellular distribution. An example of (2) is afforded by the action of Sendai virus on cultured cells; passive permeability is increased10–14 and cells swell (unpublished results of K.J.M. and C.A.P.). We show here that in both (1) and (2) the increase in surface area can be accounted for by an ‘unfolding’ of microvilli.