The osmotic behaviour of the acrosome in intact guinea-pig sperm has been examined by light and electron microscopy. Because the acrosome is retained within the cell, it can only experience the changes in tonicity of the cytoplasm and these have first to be related to the changes in tonicity in the external medium. Nevertheless, a qualitative description of the osmotic properties of the acrosome can be obtained. The evidence suggests that the acrosomal contents are normally close to their limit of compression. It has been proposed that a hydrostatic pressure is exerted inwards on the acrosomal membrane and that this is responsible for the maintenance of acrosomal shape. Swelling of the acrosome when sperm are suspended in hypotonic medium suggests that the acrosomal membrane does not have an abnormally low water permeability. Two-thirds of the acrosomal volume are occupied by a single matrix which undergoes cavitation in the course of the acrosome reaction. It also undergoes cavitation when guinea-pig sperm are suspended in calcium-free medium containing the non-ionic detergent Triton X-100. This suggests that cavitation is caused by the loss of integrity of the plasma and acrosomal membranes. A mechanism for cavitation is proposed in which a colloid osmotic pressure within the acrosomal matrix is allowed expression when the crystalloids of the external medium pass into it, this internal colloid osmotic pressure forcing the matrix apart to produce a cavity.