The ratio of the plasma of total tryptophan to those amino acids that compete with tryptophan during transport into the brain was determined in 60 control subjects and 87 patients suffering from endogenous depression, all females. The plasma ratio in the control subjects showed a significant negative correlation with age. There was no significant difference in the distribution of the biochemical data between the control subjects and the depressed patients. There was a significant higher proportion of bipolar depressed subjects compared to unipolar depressives and patients of uncertain polarity who showed a plasma ratio in the lower normal range. Thirty-two patients were subsequently treated with L-tryptophan. In the patients who showed a particularly low plasma ratio of tryptophan to competing amino acids a remission frequency of 80% was observed on day 14. The efficacy of L-tryptophan in the patients who showed a plasma ratio within the upper normal range was extremely poor. The results suggest that the ratio in the plasma of tryptophan to competing amino acids is a useful predictor of the course of treatment of depressed subjects with L-tryptophan.