The abnormality in catecholamine metabolism in the brain of SHR has been examined by use of in vivo voltammetry technique. In vivo release of dopamine and serotonin in the striatum under acute stress was more prominent in SHR at 4 weeks of age than in normotensive Wistar-Kyoto rats (WKY). Acute activation after stress of in vitro tyrosine hydroxylase activity was observed only in SHR, but tryptophan hydroxylase activity did not change significantly. It is concluded that central monoaminergic neurons are more susceptible to stress in SHR than those in WKY.