Activin is known to induce axial mesoderm during early development in Xenopus embryo. Activin receptor was recently identified to be a member of transmembrane serine/threonine kinase family. We have studied the role of activin-mediated signaling in the limb morphogenesis by identifying the target cells. We isolated cDNAs encoding chicken activin receptors, cARIIA and cARIIB, and examined their expression patterns during chick embryogenesis. The cARIIA gene is expressed in the apical ectoderm of the limb bud at stage 20-21, whereas the cARIIB gene is expressed uniformly in the limb mesenchyme. Expression of the cARIIA gene is confined to dorsal and ventral mesenchyme at stage 23, and later confined to precartilaginous cells. Transcripts of the cARIIA gene are found in developing neuroepithelium of spinal cord, brain and eye, surface ectoderm differentiating to epidermis, and myoblasts differentiating to muscle. The IIB receptor gene is highly expressed in the developing brain. These results suggest that the activins and their receptors are implicated in the limb development, especially, in differentiation of muscle, skin and bone.