Ultrastructural studies of single and serial sections of bullfrog olfactory axons showed that smooth endoplasmic reticulum (SER) tubules, which usually appear as single profiles in cross-sections of axons, are continuous over considerable distances, but that discontinuities do exist. Computer-assisted three-dimensional reconstruction of portions of axons indicated that the SER tubules show considerable variation in the volume of the cisternal space along the tubule, which often follows a tortuous path. Some branching and anastomosing appears to occur, and electron-dense material was present in the cisternal space of some tubules. SER tubules are often bridged to neurofilaments and less often to microtubules. The usual two to three microtubules in the axoplasm form a domain which is characterized by a clear area, or zone of exclusion, around the microtubules. Ultrastructural cytochemistry was used to demonstrate that SER tubules actively sequester Ca. The electron-dense product (calcium oxalate) was uniformly and specifically associated with the SER of axons at both proximal (closest to the perikarya in the olfactory epithelium) and distal (closest to the olfactory lobe of the brain) ends of the olfactory nerve. It is concluded that the primary function of SER tubules in these axons is to serve in the regulation of Ca in the axoplasm, probably to facilitate fast axoplasmic transport, and that a secondary function may be the translocation of material in the cisternal space. The observations are discussed as they may relate to the "microstream" hypothesis of axoplasmic transport, and it is argued that fast transport occurs through the zone of exclusion associated with the microtubule domain(s) of axons.