Chick embryos were paralysed in ovo with a neuromuscular blocking agent between 8 and 20 days of incubation. To evaluate the rôle of muscular activity in the development of sutural articulations, sutures of the cranial vault of control and paralysed embryos were studied histologically and the findings compared with the effect of the agent on the development of the ankle joint and some synovial joints of the jaws. Paralysed embryos showed a consistent lack of development of joint cavities in synovial joints. In most embryos, fusion of opposing cartilaginous elements had occurred. In contrast to synovial joints, sutural articulation showed a micromorphology comparable to that of controls. The findings indicate that different embryonic factors regulate the development of sutural and synovial articulations. Movements of neuromuscular origin play no essential role in the morphogenetic development of sutures, but are a prerequisite for the formation of joint cavities and other specialised structures of synovial joints.