Morphine increased levels of acetylcholine in mouse striatum in a dose-dependent manner, the increase occurring at the lowest dose previously found to produce analgesia and coinciding with the time of peak analgesic effect. Naloxone blocked this increase. After repeated injections of high doses of morphine, no effect was seen. The hippocampus was the only other brain region showing an effect, and this after a high dose. In the rat, morphine (30 and 90 mg/kg) increased striatal acetylcholine levels. At these and lower doses (2.5 mg/kg and 10 mg/kg) the ratios in the striatum of levels of acetylcholine to levels of dopamine were significantly increased. Only at the highest dose did morphine increase the levels of dopamine in the striatum and of acetylcholine in the hippocampus. Morphine did not change the levels of dopamine in the striatum and of acetylcholine in the hippocampus. Morphine did not change the levels of norepinephrine in either the hypothalamus or cortex of the rat.