In the conventional pig industry, estrus detection generally relies on behavioral symptoms and the appearance of the external genitalia, which, in most situations, is adequate. When semen is of limited quantity or quality or has been frozen, sexed or otherwise manipulated, optimal timing of insemination relative to ovulation is critical [[1], [2], [3], [4], [5]]. For in vitro fertilization, it is advantageous to have oocytes available that have been matured in vivo [[6], [7], [8], [9]]. Many studies on oocyte maturation and timing of ovulation in pigs have been based on females that had been pretreated with exogenous hormones, usually allyltrenbolone and/or human chorionic gonadotropin [[10], [11], [12], [13], [14]], equine chorionic gonadotropin [15,16] or a GnRH agonist [17]. Knox [18] has reviewed recent studies in this context. A comprehensive description of the natural course of events in the pig, however, is lacking. Moreover, there are very few studies on in vivo oocyte maturation in pigs at a molecular level [9]. The intention of the present study was to describe the natural train of events from the spontaneously occurring LH surge to the resulting cytological and molecular ovarian events and the onset of behavioral estrus.