Astrocytes obtained from the cerebral hemisphere were maintained in serum-free defined medium containing dibutyryl cyclic AMP (dbcAMP) for 4 h, at which time almost all astrocytes showed a process-bearing stellate shape. They were then exposed to methylmercury (MeHg) at 1–3 μM or solvent alone (control) for up to 24 h. Astrocytes showed a flat polygonal shape after MeHg exposure at 3 µM for 1 h or at 2 µM for 3 h, whereas the shape was not affected after exposure at less than 1 µM for 24 h. Mercury accumulation levels were similar after MeHg exposure at 3 µM for 1 h and at 2 µM for 3 h, while the level after exposure at 1 µM did not reach those levels. The shape of astrocytes exposed to MeHg at 2 µM returned to stellate from polygonal after at least 24 h, although it remained polygonal in astrocytes exposed to MeHg at 3 µM. The viable cell number was significantly lower than in the control culture only in the culture exposed to MeHg at 3 µM for 24 h. In addition, genistein, a tyrosine kinase inhibitor, inhibited the morphological changes (spreading) induced by MeHg at 2–3 µM. These results suggest that a dose-dependent difference is observed in shape changes and cell death caused by MeHg in astrocytes, and that the responses to MeHg correlate to its accumulation levels, especially for the first few hours. They also suggest that tyrosine phosphorylation may play an important role in MeHg-induced spreading in astrocytes.