From fetal rat skulls, tissue containing the presumptive coronal suture was excised and transplanted onto the exposed dura mater of adult rats. Donor animals were sacrificed at specific ages determined by developmental stages of the suture as occurs in the rat. Thus, sacrifices were made at (a) the 19th day of fetal life when a blastema formation in the suture is not yet observable; (b) the 20th day when blastema formation becomes apparent, and (c) the 21st day when the blastema is clearly visible. From the results of these experiments, the following conclusions could be drawn: (1) transplants from 19- and 20-day-old fetuses are not capable of autonomous development in host tissue; continuation and regulation of this process occur in situ only; (2) within the connective tissue of the 19- and 20-day transplants the process of suture formation is arrested; instead chondrogenetic activity occurs, resulting in the production of ectopic cartilage, and (3) when formation of the blastema has been completed prior to transplantation, i.e. at the 21st day, its capability of producing a suture in the host tissue remains unimpaired. Sutures originating from these transplants do not ossify, not even when left in situ for 2 or 3 weeks. These findings suggest the existence of an osteogenesis-inhibiting mechanism located in embryonic sutural tissue, being transmitted to the developing dura.