A recent issue of Acta Psychiatrica Scandinavica included four papers that employed the acute tryptophan depletion (ATD) technique in patients (1-4). These studies provided interesting insight into how low serotonin synthesis might influence measures of cognition, attention and aggression in young and adult patients with attention deficit hyperactivity disorder (ADHD). The studies raise two issues related to the methodology and interpretation of ATD studies. All four studies depleted tryptophan using a mixture of seven essential amino acids that lacked tryptophan. The amino acids induce protein synthesis and tryptophan levels fall as it is incorporated into protein. The mixture used did not contain histidine. As discussed in detail in a recent review (5), i) histidine is an essential amino acid, ii) in rodents variations in brain histidine levels have a greater effect on brain histamine synthesis than variations in tryptophan levels have on brain serotonin synthesis and iii) in humans acute histidine depletion with an amino acid mixture containing all the essential amino acids except for histidine lowers histidine levels and has cognitive effects. Thus, the mixture used in the studies may have resulted in decreases in both serotonin and histamine synthesis. The control mixture also contained no histidine and would have resulted in a decrease in histamine synthesis. Therefore, if there were no interaction between the effects of histidine and tryptophan depletion, the results would be valid. However, whether the effects of histidine and tryptophan depletion interact is not known. The amino acid mixture used suffers from a disadvantage not shared by other commonly used amino acid mixtures, and the lack of histidine in the mixtures was a limitation of the studies.Two of the studies found no effect of ATD. One of the papers (3) states 'ATD allows for the measurement of the influence of a real central nervous serotonergic dysfunction', while the other (1) concludes 'there is no clear evidence that short-term changes in central nervous serotonin function as induced by ATD contribute to the difficulties in processing affective prosody'. Thus, both imply that ATD is decreasing serotonin function. A decrease in function would mean that the decrease in serotonin synthesis is decreasing the release of serotonin onto postsynaptic receptors. As discussed in detail a recent review (5), there is no evidence that ATD does always decrease serotonin release. A decrease in serotonin release after ATD can be demonstrated in experimental animals, but the relevance of such animal studies to the far more complex human brain is uncertain. When ATD does have effects in humans, a decrease in serotonin release and function is usually the most plausible explanation. However, serotonin function is regulated differently in different brain areas, and because ATD decreases serotonin function in one brain area, as indicated by a particular effect that does not necessarily imply that there is a decline in serotonin function in other brain areas. T...