Berberine, a protoberberine isoquinoline alkaloid, showed inhibitory effects on dopamine content in PC12 cells $(53.8\%;inhibition;at;20;{mu}M)$. Tyrosine hydroxylase, the rate-limiting enzyme in the catecholamine biosynthetic pathway, was inhibited at $20;{mu}M)$ of berberine by $21.8\%$ relative to control. Thus, we hypothesized that the inhibition of tyrosine hydroxylase by berberine might be partially contributed to the decrease in dopamine content in PC12 cells. Furthermore, we investigated the effects of berberine on catecholamine content of serum after immobilization and cold stress in mice. Adult male mice were either subjected to 30 min of restraint or to 2 hr of cold chamber at $4-6^{circ}C$. Serum norepinephrine, 16.8 pmol/ml, in control mice was increased to 28.8 pmol/ml by immobilization and the stress-induced rise in serum norepinephrine was partially blocked by the treatment of berberine (10 mg/kg, i.p.) once a day for 6 days. Berberine (10 mg/kg/day for 3 days, i.p.) also inhibited the increase in serum norepinephrine by cold stress in mice. These results suggest that berberine may be developed as the promising antistress agent.