Boar spermatozoa were prepared for intracytoplasmic sperm injection (ICSI) by two different treatments to facilitate sperm chromatin decondensation and improve fertilisation rates after ICSI in pigs: spermatozoa were either frozen and thawed without cryoprotectants, or treated with progesterone. Morphological changes of the sperm heads after the treatments were examined and then the activation of oocytes and the transformation of the sperm nucleus following ICSI were assessed. After freezing and thawing, the plasma membrane and acrosomal contents over the apical region of sperm head were lost in all the spermatozoa. Following treatment with 1 mg/ml progesterone, the acrosome reaction was induced in 61% of spermatozoa. After injection of three types of spermatozoa, non-treated spermatozoa and progesterone-treated (i.e. acrosome-reacted) spermatozoa induced oocyte activation, but frozen-thawed spermatozoa induced oocyte activation at a significantly lower rate. Sixty-two per cent of sperm heads remained orcein-negative for 6 h, however, resulting in delayed sperm chromatin decondensation and low male pronuclear formation in the oocytes injected with a non-treated spermatazoon. Since the treatments of freezing and thawing and progesterone for spermatozoa accelerated the initial change in sperm chromatin and the latter treatment induced oocyte activation earlier, it is considered that the delay in oocyte activation and decondensation of sperm chromatin after injection of non-treated spermatozoa is caused by the existence of the sperm plasma membrane. These results show that progesterone treatment efficiently induces the acrosome reaction in boar spermatozoa without destroying their potency for oocyte activation, and the induction of the acrosome reaction results in the promotion of male pronuclear formation after ICSI.