A rapid and sensitive method for measuring 5‐hydroxytryptamine and 5‐hydroxyindoleacetic acid, using o‐phthalaldehyde and L‐cysteine, is presented, enabling both compounds to be measured in small areas of rat brain.

Summary1. In agreement with previous findings on whole brain, the intraperitoneal injection of hydrocortisone, DL-a-methyltryptophan 6. The order of the percentage decreases in the concentrations of 5-HIAA 6 h after hydrocortisone injection was, in decreasing order: hypothalamus, striatum, cerebellum, mid-brain, pons +medulla and cortex. The percentage increases after immobilization for 5 h were in the reverse order. 7. The differences between the percentage decreases in the concentration of 5-HIAA after hydrocortisone and the percentage increases after immobilization were very similar in all regions except the hypothalamus. This is consistent with immobilization stress increasing the firing rate of 5-hydroxytryptaminergic neurones similarly in different regions.8. During the first 3 h of immobilization the concentrations of 5-HIAA in the hypothalamus and in the rest of the brain increased approximately in parallel. Between 3 and 5 h, 5-HIAA returned to control concentrations in the hypothalamus while continuing to rise in the rest of the brain.9. Relative changes in the concentration of 5-HT in particulate and supernatant fractions after the various treatments were comparable except 2 h * Present address: