SUMMARY The arginine analogue canavanine strongly inhibited the replication of herpes simplex virus in BSC-I cells. Virus absorption and entry of the virus DNA into the cell nucleus were unaffected. However, changes qualitatively similar to those produced by arginine deprivation did occur. There was little inhibition of protein synthesis but the transport of proteins from cytoplasm to nucleus was substan- tially reduced. Virus DNA synthesis which was extremely sensitive early in the growth cycle was little affected at late times although virus maturation was still prevented. Possible modes of action of the drug are discussed.