Cultures of rat embryo cells were partially synchronized by the use of amethopterin and adenosine, thymidine, or deoxyguanosine and infected with H-1 virus in the G2 period and at various intervals during the S period. Infection at the beginning of the S period led to the production after 1 or 2 days of considerably more virus than did infection later in the S period or in the G2 period. It is concluded that this small DNA virus probably uses the DNA-synthesizing system of the host for its own replication. Cells infected in the G2 period were not able to reach an S period under the conditions used. The factor or factors operating against virus production in the G2 period have not yet been identified.