The follicular stigma that develops near the time of ovulation in sheep is void of thecal blood vessels. It was hypothesized that programmed apoptotic death of endothelial cells is a causative factor in this phenomenon. Apoptosis is a mode of physiological cell deletion that occurs during tissue regression and remodelling; it is characterized by oligonucleosomal fragmentation and cytoplasmic shrinkage. Direct fluorescence detection of digoxigenin end-labelled genomic DNA in situ was used as a marker of endothelial apoptosis in cross-sections of the preovulatory ovine follicular wall. There was an abrupt increase in reactive cells after the gonadotrophin surge that was restricted to the presumptive ovulatory site immediately juxtaposed to the ovarian surface (the basal follicular endothelia remained unaffected). Vascular collapse and extravasation of blood cells preceded follicular rupture. Localized damage to thecal blood vessels and haemorrhagic tissue ischaemia may be contributing determinants of follicular instability and ovulation.