1 The psychotropic effects of a single oral dose of (‐)‐tryptophan (5 g) in human volunteers were investigated using a series of physiological and psychological tests. 2 Self‐ratings of mood showed increase in drowsiness but no euphoria was detected. 3 Severe initial nausea occurred and headache increased; other bodily symptoms were unaffected. 4 Trptophan caused increased activity in the slow wavebands of the EEG but did not alter the other physiological measures. 5 The levels of total and free tryptophan in the plasma increased 8 and 20 fold respectively to peak levels 2 h after ingestion.