The majority of DNA polymerase alpha and primase activities bound to the nuclear matrix of regenerating rat liver were released into an extract by a mild sonication procedure. During maximal in vivo replication (22-h posthepatectomy) most of the solubilized alpha-polymerase and primase cosedimented at approximately 100 and 150 S as discrete megacomplexes with smaller amounts at 10 and 17 S. In contrast, high salt extracts obtained during nuclear matrix isolation as well as matrix extracts prepared just before the onset of in vivo replication (14-h posthepatectomy) were completely devoid of megacomplexes. In vitro incubation of the matrix extracts resulted in rapid dissolution of the megacomplexes to the 10 and 17 S forms. These relationships lead us to propose a dynamic assembly of the eucaryotic replisome which is initiated pre-replicatively as 10 and 17 S complexes and functionally expressed during in vivo replication as 100 and 150 S megacomplexes or "clustersomes."