AbstractWhole rabbit spermatozoa and isolated sperm nuclei were microinjected directly into the ooplasm of hamster and rabbit ova. These injected sperm decondensed and formed male pronuclei during subsequent in‐vitro culture. Injection of whole spermatozoa and sperm nuclei prepared by a protocol known to allow in‐vitro capacitation of ejaculated spermatozoa yielded a significantly higher (P<0.01) number of activated rabbit ova containing male pronuclei than did injection of uncapacitated epididymal sperm nuclei or ejaculated sperm nuclei. Rabbit ova fertilized by sperm injection were capable of undergoing normal‐appearing cleavage division during 22 h of cu