Adult rats were submitted to a 4-day starvation period or maintained on a 50% carbohydrate-restricted diet for 8 consecutive days to obtain a body weight loss of 20-30%. Serum dopamine-beta-hydroxylase (DBH) activity and amino acids content were measured as well as brain tryptophan and tyrosine levels. Moreover, brain serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), noradrenaline (NA), and dopamine (DA) contents were assayed in five brain areas. In 4-day starved and 8-day carbohydrate-restricted rats, the serum tyrosine and total tryptophan contents as well as tyrosine to the sum of six neutral amino acids ratios were lowered. Moreover, in these groups, free tryptophan to the sum of six neutral amino acids ratio remained normal and serum DBH activity increased. In the brain, to a decreased tyrosine content observed in 4-day starved and 8-day carbohydrate-restricted rats corresponded a high DA to NA ratio in the hypothalamus, thalamus, and raphe nuclei, thus suggesting a low DA utilization whereas a low DA to NA ratio was found in the neostriatum. On the other hand, brain tryptophan content was decreased in 4-day starved rats and increased in 8-day carbohydrate-restricted rats. In the former group, a high 5-HT to 5-HIAA ratio characteristic of a low 5-HT utilization was found in the hypothalamus and neostriatum whereas in the latter group a significant decrease in this ratio was only observed in the thalamus. These results suggest that the biochemical response to starvation vs carbohydrate restriction can be differentiated on neurochemical and neuroanatomical bases.