Mucous granules secreted by the slime glands of the hagfish. Eptatretus stouti, were studied after ultrarapid cryofixation and freeze substitution in diverse media (osmium tetroxide in acetone; several aqueous glutaraldehyde-based media with or without osmium). Only freeze substitution with osmium tetroxide-acetone preserved the granules intact, allowing visualization of its single unit membrane. Tests of the rupture or stability of freshly secreted mucous granules in sea water and other aqueous media showed the membranes of the granules are permeable to all inorganic cations tested, ranging in relative mass from ammonium to barium. They are permeable to the univalent anions chloride, nitrate, and bicarbonate, but not to the di- or trivalent anions sulfate, phosphate, and citrate. Moreover, in solutions where nonpenetrant anions were present, rupture occurred if the osmotic pressure was below a critical level (about 800 mOsmol/l). The structural and permeability characteristics of the granules account for the explosive speed with which they rupture, releasing their mucous contents, on contact with sea water.