When female voles were allowed contact with the stud male for only 1 h at the time of mating, 55% exhibited pregnancy failure when exposed to a strange male 48 h later. When females were made psuedopregnant by hormone treatment and vaginal stimulation (i.e. no stud male involved), 87% exhibited luteal failure when exposed to a strange male. It is suggested that the characteristics of the stud male are rapidly imprinted upon the female at the time of mating and that this imprinting is important in preventing the female showing a blocking response to this male upon any subsequent exposure.