Abstract— The turnover of 5-hydroxytryptamine in the forebrain and of dopamine in the striatum was studied in mice fasted for 20 h. Such mice showed an increased tissue concentration of 5-hydroxyindoleacetic acid in the forebrain and an increased accumulation of this acid after probenecid. Fasted mice also showed a higher concentration of homovanillic acid in the striatum than fed mice. However, the administration of probenecid produced a smaller increase in homovanillic acid concentration in fasted than in fed mice. The decay of dopamine following α-methyl-p-tyrosine was reduced in fasted mice at 2 h, but not at 1 h or 6 h after administration of the inhibitor. The possibility that fasting increases the activity of some dopaminergic neurones while decreasing the activity of others is considered. The existence of a pool of homovanillic acid at a site within the striatum where the probenecid-sensitive transport is not effective is postulated.