Oocytes collected in stimulated cycles are more readily fertilized after preincubation than are oocytes inseminated immediately after collection. It has not been ascertained, however, whether this increase in the fertilization rate is due to extrusion of the first polar body (meiotic maturation) during this period, or to some conclusive cytoplasmic maturation subsequent to the polar body extrusion. When analysing oocytes with a polar body (n = 14) by transmission electron microscopy, differences were observed in the appearance of cytoplasmic features which were correlated to the total durations both of systemic human chorionic gonadotrophin influence before collection and of oocyte culture. These differences were manifested as a delayed formation in aggregates of the smooth endoplasmic reticulum in the ooplasm of oocytes having a polar body and might have signified a cytoplasmic maturation. The degree of asynchrony in the oocytes varied and this could explain why some oocytes can be fertilized when inseminated shortly after collection and others not until 8 h or even more after collection. Thus, although meiotic and cytoplasmic maturation is likely to be synchronized at ovulation of an oocyte in a natural cycle, they appear to be mainly asynchronous in oocytes collected in stimulated cycles.