The effect of altering normal cell associations and interactions on the synthesis of 5S RNA and transfer RNA (tRNA) was studied in cleaving embryos of the sea urchin, Arbacia punctulata. Cell interactions were altered: (1) by culturing cleaving embryos in the animalizing agent, Evans Blue, and in the vegetalizing agent, Li+ as LiCl and (2) by culturing dissociated cells. Control and experimental embryos each were labeled from 3 h to 6 h post fertilization with [8-3H]-guanosine. Sixteen-cell embryos, whose GTP precursor pools had been preloaded, were dissociated, labeled and cultured under conditions which prevent reaggregation. Quantitative measurements of rates of accumulation of newly synthesized 5S RNA and tRNA showed that these rates are similar in cleaving sea urchin embryos and in corresponding embryos cultured in the presence of Evans Blue and of Li+. In addition, cells dissociated from cleavage embryos and maintained under conditions which prevent reaggregation retained the ability to synthesize 5S RNA and tRNA. These results suggest that normal cell associations and interactions are not necessary for the synthesis of 5S RNA and tRNA to occur in cleaving sea urchin embryos.