Replication complexes were reconstituted using the eight purified bacteriophage T4 replication proteins and synthetic circular 70-, 120- or 240-nt DNA substrates annealed to a leading-strand primer. To differentiate leading strands from lagging strands, the circular parts of the substrates lacked dCMP; thus, no dCTP was required for leading-strand synthesis and no dGTP for lagging-strand synthesis. The size of the substrates was crucial, the longer substrates supporting much more DNA synthesis. Leading and lagging strands were synthesized in a coupled manner. Specifically targeting leading-strand synthesis by decreasing the concentration of dGTP decreased the rate of extension of leading strands. However, blocking lagging-strand synthesis by lowering the dCTP concentration, by omitting dCTP altogether, by adding ddCTP, or with a single abasic site had no immediate effect on the rate of extension of leading strands.