The effect of morphine (10 mg/kg, s.c.) on the rate of [3H]5-HT synthesis in brain and spinal cord following intravenous injection of [2H]tryptophan was studied in the rat. (1) In the spinal cord morphine induced an increase in [3H]TRP uptake which was significant 30 and 60 min after the injection, and a clear enhancement of the formation of [3H]5-HT which was significant 15 min after the injection and which peaked at 30 min. In the forebrain, the increase in [3H]TRP accumulation was significant as early as 15 min after morphine. At this time, the rate of 5-HT synthesis was not modified, but it was significantly increased 30 and 60 min after the injection. (2) Increases in [3H]5-HT synthesis rate were suppressed by naloxone (1 mg/kg, i.m.). However this narcotic antagonist did not significantly reduce the increased accumulation of [3H]TRP in brain and spinal cord due to morphine treatment. These results demonstrate that morphine induces a fast and marked increase in 5-HT synthesis, and suggest that this increase is only partly related to an increase in the availability of tryptophan to the central nervous system. These results are in good agreement with recent investigations showing the involvement of the raphe-spinal system in morphine analgesia.