The effects of acute tryptophan depletion were investigated in 20 healthy volunteers. Ten of them received a balanced amino acid solution and 10 a tryptophan-free solution. The fall in tryptophan levels induced by the oral administration of a mixture tryptophan-free of L-amino acids was - 77% for free tryptophan and - 81% for total tryptophan. Before treatment, there were intergroup differences affecting alertness parameters (critical flicker threshold, recognition reaction time), plasma levels of prolactin and baseline performance in the proofreading correction test. In the tryptophan-depleted group, the number of errors reported during the unpleasant sound signal was increased (+ 48%) after treatment, whereas the number of errors fell (— 15%) in the group receiving the tryptophan supplement. Conversely the levels of prolactin were correlated with those of serum tryptophan. Few subjective effects were reported.
