Electron microscopy of hepatic cells from protein-deficient young rats revealed extensive breakdown of the rough endoplasmic reticulum into fragments and vesicles whose membranes appeared fully studded with ribosomes. Additionally, there was biochemical evidence of marked disaggregation of polysomes (n > 2), and this was more prominent in the membrane-bound than in free polysomes. Protein malnutrition increased the proportion of membrane-bound ribosomes which were salt-releasable and therefore temporarily non-functional. It has been concluded that disaggregation of membrane-bound polysomes induced by malnutrition does not necessarily imply detachment of the monomeric ribosomes from the endoplasmic membrane into the pool of free ribosomes, which probably has a separate polyribosomal cycle.