The mouse osteoblastic MC3T3-E1 cells were cultured for 7-10days before subconfluence and then placed in serum-free medium for 24 hours and the cells were subsequently treated with 1.53mM Ca²⁺, calcium ionophore 2pM A23187 and Ca²⁺+A23187, for 30 minutes. The c-fos mRNA expression in MC3T3-E1 cells is induced strongly in normal medium (1.87mM Ca²⁺ ; control group) but slightly in low calcium medium (0.34mM Ca²⁺; low Ca group) by A23187. The degree of expression with Ca²⁺ or Ca²⁺+A23187 treatment seemed to be higher in the low Ca group than in the control group. On the other hand, Ca²⁺+A23187 suppressed induction of c-fos mRNA expression with the A23187 treatment in the control group. Due to the above, the following was concluded. First, the stimulation of c-fos mRNA expression by A 23187 occurs only in the presence of extracellular calcium. Second, the MC3T3-E1 cells placed in a low calcium environment react to restore normal cell function at the gene level, mediating Ca²⁺ which plays a role in the induction of c-fos mRNA. Third, in a high calcium environment, the activity of an intracellular signal transduction system for c-fos gene function seemed to be suppressed.