We investigated the time course of human oocyte activation after intracytoplasmic sperm injection (ICSI) by observing the oocyte chromosome configuration at different times after injection. One day old human oocytes were injected with spermatozoa and subjected to cytogenetic analysis at 2, 3, 4 and 5 h after injection. We found that anaphase is initiated in the vast majority of the oocytes between 2 and 3 h after injection, and that by 4-5 h after injection most of the oocytes have reached the chromatin mass stage. Two distinguishable stages of sperm nucleus transformation were observed. The first phase-swelling-was reached within 2 h after the injection and was independent of oocyte activation. The second phase-the "brush'-like stage or decondensed chromatin stage-was found only in activated oocytes. Moreover, this stage was not reached before the chromatin mass stage (late telophase) of the oocyte. The same proportion of metaphase II oocyte chromosome configurations and unchanged sperm nuclei was found at any given time after injection. We conclude that: (i) ICSI allows users to obtain an almost synchronized population of activated oocytes; (ii) anaphase II is initiated in the majority of oocytes not later than 2-3 h after injection and telophase II is reached approximately 5 h after injection; and (iii) there are two distinguishable phases of sperm nucleus transformation after ICSI: oocyte activation-independent swelling of the sperm head and oocyte activation-dependent chromatin decondensation which is coupled to the beginning of oocyte chromosome decondensation.