Essential tremor (ET) is one of the most common movement disorders, exhibiting a postural and/or kinetic tremor. We previously analyzed tremorgenic gene components using a ET model, Tremor rats, and showed that deletion of aspartoacylase (Aspa) and a missense mutation of Hcn1 channels are implicated in induction of tremor (Exp. Anim. 65, 293–301, 2016). Since monoamines are known to be involved in regulation of tremor induction, here, we evaluated the changes in brain monoamine levels in Aspa-knockout (KO) rats. Dopamine, 5-HT and their metabolites were analyzed in 9 brain regions (cerebral cortex, hippocampus, striatum, thalamus, hypothalamus, midbrain, inferior olive, pons and cerebellum), using a HPLC-ECD method. Aspa-KO rats showed increases in 5-HT levels in the hypothalamus and inferior olive, and in 5-HIAA and 5-HIAA/5-HT levels, in most brain regions examined, indicating that deletion of Aspa gene enhances the 5-HT turnover rates. Alterations in HVA levels were also found some regions (increases: cortex and striatum, decreases: hippocampus and hypothalamus). The present results suggest that enhancement 5-HT synthesis and metabolism, especially in the inferior olive, may be linked to tremor induction.