Effects of intracerebral (i.c.) administration of metaraminol (MA) on brain monoamine in mice were studied. The results were as follows: MA, dopamine (DA), noradrenaline (NA) and 5-hydroxytryptamine (5-HT) were separated with phosphate buffer using a phosphorylated cellulose column (1X11 cm, 9 ml, Lot No. 2809). When MA was i.c. injected into mice, fluorescence of MA at 275/305 nm decreased in proportion to the time course and remained even at 12 hr after injection. Thirty min after MA in doses of either 40 or 80 microgram, a significant decrease of DA as compared with that of saline-treated group occurred and MA, in doses of 40, 80 and 160 microgram produced a significant decrease of NA and 5-HT. Thirty min after 160 microgram of MA, there was no significant difference in DA. Three hr after MA, DA levels significantly decreased, and 168 hr after were restored to the levels of the saline-treated group. Thirty min after MA, NA and 5-HT signficantly decreased compared with those of the saline-treated group and recovery took place 48 hr and 168 hr after respectively. It was concluded that MA depletes not only NA but also DA and 5-HT in the mouse brain.