It has recently been suggested that RhoA plays an important role in the enhancement of Ca2+ sensitization observed in smooth muscle contraction. In the present study, the expression of rhoA mRNA in the bronchial smooth muscle of antigen-induced airway hyperresponsive rats was compared with that of control animals. Reverse transcription-polymerase chain reaction experiments using total RNA from these tissue specimens and the specific primers revealed rhoA mRNA to be expressed in bronchial smooth muscle of the rat. The rhoA mRNA expression in bronchial smooth muscle of the hyperresponsive rats was significantly increased in comparison to that of control animals. It is thus possible that upregulation of RhoA protein might be involved in the mechanism underlying the increased contractility of the bronchial smooth muscle which occurs with airway hyperresponsiveness.