The acrosome is a secretory vesicle located in the sperm head. The acrosome reaction consists in the fusion of the sperm plasma membrane with the external acrosomal membrane. It has been observed that this reaction does not take place in spermatozoa incubated in cervical mucus, hydrogel that contains high concentrations of oestradiol in the peri-ovulatory period. The objective of the present study was to analyse the influence of oestradiol on the acrosome reaction in human spermatozoa to evaluate the possible inhibitory effect of this hormone. Spermatozoa were incubated in progesterone (10.1 nmol l(-1)); oestradiol plus progesterone (oestradiol at 840 pmol l(-1) and progesterone at 10.1 nmol l(-1)), oestradiol (840 pmol l(-1)) and control (without steroidal hormones) for 30 min, 60 min, 240 min and 24 h. The acrosome reaction was evaluated by stain with Hoechst 33258 and fluorescein isothiocyanate-conjugated Pisum sativum agglutinin lectin. Progesterone-incubated spermatozoa showed the highest percentage of acrosome reaction (P