The ultrastructural morphology of ribosomes was studied in tissue sections of rat uterus using defferent fixatives (acrolein formaldehyde, acetic acid, methanol-acetic acid, OsO4) after various pre-incubations in solutions of different osmolarity, electrolyte content, and pH. In addition, whole-mount ribosomes of spread cytoplasm of epithelial cells were examined. The results indicate: Ribosomes are "thickenings" in the course of a cytoplasmic RNP-network which extends in three dimensions between nucleus and plasma membranes. This network consists of fibrils which vary in width depending on intrinsic and extrinsic factors. Often the fibril width is approx. 100 A. In places where the fibril is folded up into ribosomal "granules" and strands the width may be 300 A. Each fibril seems to be composed of two elementary filaments, 10 A up to 40 A thick. Estimation of filament length within a ribosomal "granule" allowed the conclusion that only a fraction of a 45 S-RNA filament is folded up into a "ribosome" in cytological preparations. No morphological evidence was found for clefts within ribosomes or single messenger RNA filaments "piercing" ribosomes. Filaments of membrane-bound ribosomes appearently show continuities with fibrils within the cytoplasmic matrix on the one side and with filaments within the cisternae of the endoplasmic reticulum on the other. Ribosomal filaments of stromal cells also seen to have filamentous continuities with extracellular fibrils. This morphological evidence opens new perspectives concerning the role of RNA filaments in protein synthesis.