Fifty consecutive patients requiring nonsurgical endodontic therapy were the subjects of this pain-control study. Twenty-five subjects received 3 gm of tryptophan in divided 0.5-gm doses over a 24-hour period, beginning before the start of treatment. The other twenty-five subjects received an identically appearing placebo on the same dosage schedule. Pain intensity was evaluated by the subjects on a scale of 0 to 10 (0 = no pain, 10 = severe pain) (1) prior to the start of treatment, (2) after 24 hours, and (3) 1 week later. Results showed that the most meaningful difference was found at 24 hours, when the tryptophan group was significantly better than the placebo group (F = 7.46, df = 1.96, p less than 0.01).