Membrane-bound ribosomes are thought to secrete protein for export and free ribosomes to secrete protein for intracellular use. The proportion of the total ribosomes that is bound to membranes in normal mouse kidneys has been estimated by three different methods, and the results have been compared with those obtained by a fourth method used by us previously. The most valid estimates appear to be those obtained (a) by comparison of radioactivity in peaks representing free and membrane-bound ribosomes on linear sucrose gradients after labeling for 24 hr with (14)C-orotic acid, and (b) by measurements of optical density in free and bound ribosomes that had been separated by centrifugation on discontinuous gradients of 0.5 M/2.0 M sucrose. Analyses by these methods show that about 20-25% of the ribosomes in a postnuclear supernatant prepared from mouse kidneys, but only 10-15% of the ribosomes in a post-mitochondrial supernatant, are membrane-bound. About 75% of the bound ribosomes sediment as polysomes of many different sizes. The proportion of membrane-bound ribosomes and their aggregation into polysomes were unchanged in kidneys undergoing compensatory hypertrophy after removal of the opposite kidney. These experiments show that, unlike liver, kidney has a predominance of free ribosomes compared to bound ribosomes; those ribosomes that are membrane-bound do not become free during compensatory renal growth.