To the Editor.
—The measurement of levels of monoamine metabolites in CSF gives some information on central neurotransmission in man. As several factors affect these levels, the procedure of the lumbar puncture must be standardized. A constant volume of CSF must be drawn, because the levels of 5-hydroxyindoleacetic acid (5-HIAA) and homovanillic acid (HVA) increase with increasing volume.1-2 This is most certainly due to a fairly steep ventriculospinal CSF gradient of these two metabolites. For the norepinephrine metabolite 3-methoxy-4-hydroxyphenyl glycol (MHPG), this gradient is much less pronounced.To investigate whether the site of the lumbar puncture also is of importance for the levels of 5-HIAA and HVA, we performed lumbar punctures twice in seven healthy volunteers (men aged 25 to 46 years). The two punctures were performed at 8 AM after at least eight hours of fasting and bed rest. With the subjects in the sitting position, 6 mL of