Isolated cells of Xenopus laevis neurulae were labeled with 3 H-uridine at three different temperatures (21°C, 30°C and 10°C), and the pattern of RNA synthesis was examined, using polyacrylamide gels. At 21°C (standard temperature), the synthesis of 28S and 18S ribosomal RNA (rRNA), as well as other RNAs, occurred quite actively. At 30°C (higher temperature), the syntheses of rRNA, 4S and heterogeneous RNA were all increased about twofold (Q10 = 2). When the temperature was lowered to 10°C, however, the accumulation of both 28S and 18S rRNA was inhibited nearly completely, while the other RNAs were still synthesized at a 50% level. The absence of 40S rRNA precursor at 10°C makes it highly likely that at the lowered temperature the transcription of rRNA precursor is inhibited. These effects of temperature changes on RNA synthesis were all reversible: the synthesis returned to a normal rate within 4 hr in those cells that had previously been exposed to the lower or higher temperature for 6 hr.