The objective of this communication was to study the effect of portacaval shunting in rats, on RNA synthesis, in five major sections of their brains. This was achieved by injecting ³H-uridine into adult male rats 1 hour, 7, 10 and 13 weeks after they had a protacaval shunting, and sacrificing them four hours after administration of the label. Uptake of tritiated-uridine, a measure of tRNA and mRNA synthesis combined, was measured in the rats` frontal cortex, corpus striatum, thalamus and hypothalamus, brain stem and cerebellum. There was significant increase in uridine uptake, in some brain sections, already seven weeks after the shunting, which was rebound (a significant decrease) towards the 10^th week after the shunting, and returned to control levels around the 13^th week after surgery. Portacaval shunting modulates the synthesis of tRNA and mRNA in rats, but the marked increase of uridine uptake in the frontal cortex and cerebellum on the 7^th week is followed by adaptive and compensatory processes, including formation of collateral circulation, which brings the RNA’s rate of synthesis to its control levels towards the 13^th week of the study.