The three-dimensional organization of the smooth endoplasmic reticulum (SER) in Purkinje cell dendrites in the chick cerebellum was investigated to assess the connectivity between its various components. Three-dimensional reconstructions of the SER within portions of Purkinje cell dendrites were performed from serial sections through the cerebellar molecular layer. In addition, semithick and thick sections (0.25-1 micron) of chick cerebellum in which the endomembrane system had been selectively stained were examined with an intermediate high-voltage electron microscope. Three-dimensional information was extracted from thick sections using stereo pairs and tomographic reconstructions from single axis tilt series. In contrast to its discontinuous appearance in thin sections, in both the three-dimensional reconstructions and selectively stained thick sections the endoplasmic reticulum formed a highly interconnected network of tubules and cisterns extending throughout the dendritic shaft and into the spines. Several distinct morphological domains of endoplasmic reticulum were noted, including the hypolemmal cisternae, the endomembranes associated with the dendritic spines, and the tubular and cisternal endoplasmic reticulum in the dendritic shaft. In older chicks (aged 2 weeks), stacks of endoplasmic reticulum were also noted within the dendritic shaft. All elements of the SER within the dendritic shaft and spine appeared to be continuous with one another. The results of this study strongly suggest that the endoplasmic reticulum forms a complicated network that may be part of a single endomembrane system within Purkinje cell dendrite.