Cell‐free extracts prepared from unfertilized eggs of Xenopus laevis support DNA synthesis on sperm pronuclei. Continuous labelling studies using [3H]dCTP and pulse labelling studies using [32P]dCTP demonstrate that synthesis occurs in short bursts of 40 min, which are punctuated by periods of 20‐40 min during which no synthesis occurs. Density substitution experiments using bromodeoxyuridine demonstrate that this synthesis involves the initiation of replication and reveals that re‐initiation events can occur following multiple bursts of replication. The periodic properties of these extracts are sensitive to protein synthesis inhibitors.