Intracytoplasmic sperm injection (ICSI) into mouse oocytes involves a very low survival rate. This study was designed to determine why ICSI frequently fails in mice. Metaphase II oocytes were obtained from superovulated 4-6 week old F1 hybrid mice. Spermatozoa were retrieved from the epididymis of 12-14 week old F1 hybrid mice. The spiked microinjection pipette used to inject a spermatozoon into the ooplasm had outer and inner diameters of 10 and 8 microns respectively. The oocytes used in the first part of the study were not activated (group 1). Some oocytes were incubated with calcium ionophore for 5 min (group 2). The injected oocytes were evaluated 6, 20, 48 and 72 h after injection. A total of 143 eggs in each group underwent ICSI. In group 1, sperm heads escaped into the perivitelline space. In all, 63 (47%) of the remaining oocytes were damaged during the injection or had degenerated by the first evaluation. The survival rate was 53%, but fertilization did not occur. In group 2, 31 oocytes (22%) were damaged during microinjection or soon degenerated. Two oocytes underwent accidental subzonal insemination. Six oocytes were fertilized (4.2%) among the 78% of survivors. After injection, the sperm tail was found in the cytoplasm (27 and 31% in groups 1 and 2 respectively), the perivitelline space (45% in both groups) or protruding through the zona pellucida (28 and 23% respectively). More oocytes degenerated when the tail remained in the cytoplasm, i.e. 78% in group 1 and 36% in group 2.