Five patients on chronic opiate medication to treat low-back and leg pain were determined to have developed opiate tolerance on the basis of their failure to obtain significant relief (rated on a subjective pain scale and by the degree of straight leg-raising they were able to endure) after receiving 30 mg of morphine administered i.v. in divided doses over 35 min. After these patients' diets had been supplemented with 4 g/day of L-tryptophan for 2-9 weeks, they achieved significant relief from pain when the opiate tolerance test was re-administered, and were able to lead more active lives while reducing their daily opiate intake. chronic opiate administration probably reduces the serotonin turnover rate in the central nervous system; it may be that this is reversed by loading with the serotonin precursor, L-tryptophan.