Brain harmine was simply and sensitively determined by high performance liquid chromatography (HPLC), and the relationship between brain harmine concentrations and the jumping behavior induced by harmine and apomorphine was investigated. The concentration of harmine and the peak height in HPLC showed a good correlation, and the detection limit was 0.05 pmole harmine. In rats treated with 2, 5 and 10 mg/kg of harmine, the concentrations of harmine in the brain cortex were 9.0, 21.3 and 43.1 nmole/g tissue (wet wt.), respectively. The regional brain harmine concentrations and the jumping activity measuring by a scoring system increased with increasing doses of harmine administered, and there was a direct relation between the harmine concentrations and the jumping activity. This relation was observed in washed membranes in which about 40% of the harmine in whole homogenate was present. Tissue subfractionation showed that 30% of the harmine in whole brain homogenates was in the P2 fraction, and of this, 70% was located in the synaptosomes. The brain homogenate did not metabolize harmine and no metabolite was detected in the brain 10 min after treatment with harmine and apomorphine. These results suggest that harmine itself, but not its metabolites, may be responsible for inducing the jumping behavior and that the jumping activity is correlated with brain harmine concentrations.