Following infection, retroviruses insert a DNA copy of their RNA genome into the host cell genome. This integrative recombination reaction occurs at specific sites on the viral DNA: inverted repeat sequences near the termini of the linear DNA form of the viral genome. We have described elsewhere the generation and analysis of deletion mutations at one of the inverted repeat sequences in Moloney murine leukemia virus. We describe here the effects of insertion mutations made at this locus. Our results show that substantial sequence changes at the site of recombination can be tolerated, and that the spacing between the cleavage sites on the viral DNA can be expanded as well as contracted while still allowing efficient viral integration. After several rounds of virus replication, each of the insertion mutants gave rise to pseudorevertants with new alterations at the integration site.