Plants express a variety of proteins at the cell surface responsible for the transduction of regulatory information into the cell via receptors. In the present study, an attempt has been made to identify the components of the brassinosteroids (BRs) signaling transduction cascades in transgenic rice (Oryza sativa) expressing the antisense strand of OsBRI1 transcript. A 60 kDa protein, immunologically characterized as mitogen-activated protein kinase (MAPK), showed reduced phosphorylation activity in the membrane fractions of the OsBRI1 antisense rice over control, demonstrating the inhibition in the perception of BRs by the BRI1 receptor, when compared with the exogenously applied brassinolide. The phosphorylation activity of the 50 kDa Ca(2+)-dependent protein kinase was however increased in the cytosolic fractions of OsBRI1 antisense over control. The data obtained suggest that MAPK and Ca(2+)-dependent protein kinase in rice are discrete but parallel signaling cascades and might involve receptors other than BRI1 in response to BR stimulus.