Abstract The polysomes of NCTC clone 929 mouse cells, strain L, decrease to one-third of the normal level upon nitrogen starvation. The decrease, which appears to be a consequence of dissociation of ribosomes from the messenger RNA, can be reversed dramatically and completely by refeeding with the growth medium. When the cells are exposed to actinomycin D (2 μg/ml) their polysomes dissociate rapidly and quantitatively. This dissociation is also readily reversed when the antibiotic is removed and the growth medium is added back to the cells. The data support the hypothesis that messenger RNA, released upon dissociation of polysomes in these cells, is relatively stable and capable of reforming polysomes when nutrient conditions have been restored.