Changes in the serotonergic nervous system after the intracerebroventricular (i.c.v.) administration of ethylcholine aziridinium (AF64A, 3 nmol/each ventricle) were studied in rats. Two weeks after the infusion of AF64A, the levels of 5-HT and 5-HIAA in microdialysed cerebrospinal fluid (CSF), the levels of total 5-HT and 5-HIAA, the density of serotonin uptake sites and the activities of tryptophan hydroxylase (TPH) and monoamine oxidase (MAO) in various brain regions were determined. After AF64A administration, the concentrations of 5-HT in lateral ventricle were increased and the levels of 5-HIAA were decreased. However, the hippocampal levels of total 5-HT were decreased without changes in the levels of 5-HIAA and the hippocampal turnover rates of 5-HT increased. Also, the density of uptake sites of serotonin ([(3)H]citalopram binding sites) was decreased in the various brain. The activities of TPH were increased in striatum and frontal cortex and the activity of MAO was also increased in striatum. These results indicate that AF64A induces an increase in serotonergic neuronal activity and decreased densities of 5-HT uptake sites which may affect the change in the other parameters of serotonergic neuronal activities. Furthermore, these results suggest that the impaired cholinergic neuronal activity induces the alteration in the serotonergic nervous activities.