Groups of young adult male rats ingested ad libitum for two weeks diets containing 2%, 5%, 10% or 20% protein. They were then killed early in the daily dark period, 30 min after receiving m-hydroxybenzylhydrazine (100 mg/kg i.p., to allow measurement of in vivo tyrosine hydroxylation rate). Tyrosine levels in retina, hypothalamus and cerebral cortex were lowest in rats ingesting 2% protein, and rose progressively to a plateau in rats ingesting 10% protein. No consistent increase occurred between 10% and 20% protein. Tyrosine hydroxylation rate in retina and hypothalamus, but not prefrontal cortex rose in parallel with the increments in tyrosine level between 2% and 10% protein, showing no further increase between 10% and 20% protein. These changes were found to be related to the level of protein, not caloric intake. And, the low rates of hydroxylation observed in rats consuming low protein (2%) were found not to be attributable to low endogenous tyrosine hydroxylase activity. Together, the results indicate that the differences in tyrosine levels in some regions of the central nervous system (retina, hypothalamus) produced by chronic variations in protein intake may influence directly the rate of tyrosine hydroxylation, and thus perhaps the overall rate of catecholamine synthesis. This relationship might provide the hypothalamus (a region important in food intake control) with a signal for monitoring and ultimately modulating the chronic level of protein intake.