Electron microscopic studies of the dentate gyrus of the rat have revealed an apparent association between polyribosomes and dendritic spines. The present study was designed to elucidate the nature of this association. Our qualitative observations revealed that polyribosomes appeared primarily in two locations within the dendrite: (1) beneath the base of identified spines just subjacent to the intersection of the spine neck with the main dendritic shaft and (2) beneath mounds in the dendritic membrane which had the appearance of the base of a spine which extended out of the plane of section. To quantitatively define the nature of the apparent association, we attempted to determine (1) the proportion of spines with associated polyribosomes and (2) the proportion of the polyribosomes within dendrites which are associated with spine bases. Evaluation of profiles which were identifiable as spine neck-dendritic shaft intersections in a single section revealed that an average of 12.2% had associated polyribosomes. A serial section analysis revealed a somewhat higher incidence, however. Of a collection of 34 through-sectioned spines, 29% had polyribosomes which were revealed in one or more of the sections comprising the series. To evaluate what proportion of polyribosomes within the dendrite was associated with spines, we evaluated a series of photographs covering approximately 1250 micrometer2 of the dentate molecular layer from five animals, identifying all polyribosomes within dendrites and scoring their location as being (1) under spines, (2) under mounds, or (3) other. An average of 9.6% of the polyribosomes were found under processes identifiable as spine neck-dendritic shaft intersections, while 71.4% of the polyribosomes were found under mounds. Only 19% were not obviously associated with spines or mounds. Spine bases and mounds comprise only 3 of 35%, respectively, of the outline of dendritic profiles, however, indicating that the high incidence of polyribosomes under these elements cannot be accounted for by chance. To attempt to determine whether the mounds represent the base of dendritic spines, 68 mounds in 21 dentritic profiles were selected from the middle of the series of 20 serial sections. Ninetine of these mounds (28%) were continuous with an identified spine, and an additional 31% were continuous with thin processes of the size and appearance of spine necks. Thus, most of the mounds probably do represent the base of spines which extend out of the plane of a single section.