1. A new substance, MP-809, a tryptamine derivative, was investigated in 50 depressed patients, as compared to two other groups of 50 depressed patients receiving inert and potent placebo, in a multiblind controlled setting.2. Clinically MP-809 had a significant (marked to moderate) antidepressant activity in 52% of all patients and in 74.1% of neurotic depressions. Statistically the results were significantly in favour of MP-809 as compared with the two placebos, and this significance reached the high level of P<0.001 in neurotic depressions.3. Side effects were frequent but minimal in their intensity.4. The final conclusion was that MP-809 is a potent antidepressant agent particularly in neurotic depressive states.