Intramuscular nerve fibres in the bladder of adult female rats were investigated by means of serial sections. The following observations were made. (1) Upon penetrating into the musculature the nerve bundles branch repeatedly, and almost all turn into single fibres; their axons become varicose, the Schwann cell sheath is attenuated, incomplete or absent, and the separation between axonal membrane and muscle cell membrane is reduced to tens of nanometres. (2) All single axons, and some of those within bundles, are varicose, but the characteristic of being varicose is expressed by degrees, and is not an all-or-none state. (3) Varicosities contain vesicles (mostly of the agranular type), microtubules (with little connection with the axolemma or the vesicles), some neurofilaments (scarce or absent in the best developed varicosities), mitochondria (whose size is on average smaller than those of the perikaryon, and a minute amount of endoplasmic reticulum. (4) Terminal varicosities, the true anatomical ending of an axon, are often devoid of Schwann cell sheath, are packed with vesicles, rarely contain microtubules or neurofilaments, and lie close to a muscle cell: the gap is often reduced to approximately 10 nm. (5) Schwann cells accompany the axons within the muscle strands. Unlike the area of the axonal profiles, the area of glial sheath changes little along the length of the nerve fibre, except towards its end. (6) The Schwann cell sheath around a varicosity is often incomplete; the area of the axolemma thus exposed is covered by the basal lamina, and is here referred to as a 'window'. While some varicosities have a window only a few tens of nanometres in width, others have more than one window, and some are devoid of Schwann cell altogether, so that their entire axolemma is in contact with the basal lamina. The Schwann cell never extends beyond the axon, whereas very often (and possibly always) the axon extends beyond the Schwann cell. (7) Intervaricose segments vary in length and diameter, the narrowest ones accompanying the more clear-cut varicosities. Some intervaricose segments are as small as 50 nm in diameter, contain a single microtubule and lack a Schwann cell sheath. Others, sheathed by a Schwann cell, contain a single neurofilament or no organelles at all.(ABSTRACT TRUNCATED AT 400 WORDS)