Primarily cultured astrocytes were treated with ammonium chloride in various concentrations, namely 5mM, 10mM and 15mM. The changes were does-and time-dependent. After 1 to 4 days of treatment, there were mild vacuolization, eosinophilic granulation, hypertrophy and hyperplasia with increasing mitotic figures and enhanced GFAP reactivity. The number of cells and mitoses were four and five times more than those of the normal controls respectively (P < 0.01). The cells displayed an increase in number and width of the processes. The mean area, perimeter as well as the form factors were significantly increased as compared with those of the control (P < 0.01). While in the later stage (7-10 days after NH4Cl treatment), lamellation and fragmentation of the cellular processes, remarkable vacuolization and granulation as well as appearance of eosinophilic bodies in the cytoplasm were obtained accompanied with vacuolated or pyknotic nuclei. The GFAP reactivity became weak even negative. The results indicated that to the administration of ammonium chloride, the astrocytes had a dynamic plasticity.