The uptake of nucleosides and the synthesis of RNA in Tetrahymena thermophila were examined following amino acid starvation. Omission of leucine, phenylalanine, or arginine from the medium resulted in a rapid decrease in the incorporation of [3H]uridine into the acid-soluble pool and acid-insoluble material (RNA). Amino acid starvation inhibited the uptake of all ribo- and deoxyribonucleosides tested but did not affect the uptake of amino acids or glucose. In addition, under the conditions used, the omission of an amino acid did not result in a large decrease in amino acid incorporation into total protein. Treatment of cells with cycloheximide or emetine gave results similar to the effects of amino acid starvation, but in these experiments the inhibition of protein synthesis was essentially complete. Nucleotide pool sizes were also measured following amino acid starvation. ATP and UTP levels were essentially unchanged, but the dTTP pool size was decreased by 40%. The decrease in RNA synthesis in vivo in the absence of an essential amino acid was reflected in the endogenous RNA synthetic activity of isolated nuclei. However, when solubilized RNA polymerase activity was measured with calf thymus DNA as template, no significant difference was observed between control and amino acid-starved cells.