In order to assess the generality of previous reports of enlarged genital sensory fields following estrogen treatment, facial receptive fields of individual trigeminal ganglion neurons from ovariectomized female rats injected with estrogen were compared with fields from noninjected controls. Mechanoreceptive field boundaries were determined with a series of stimulating probes varying in diameter, and calibrated according to the force delivered at the bending point (Von Frey technique). For each single unit, the threshold receptive field was determined with the smallest stimulus probe capable of reliably activating single unit discharges. Facial receptive fields from estrogen treated females were significantly larger in area (by a factor of 10) than fields from noninjected controls. Furthermore, the increase in field area was independent of spatial location within the facial region (normally, field size decreases in a proximal distal direction from the body trunk), and was not affected by cutting the trigeminal nerve root. Receptive field thresholds were also significantly lowered by estrogen treatment. The fact that long term systemic estrogen treatment influences tactile sensory fields in the facial region of females suggests that this phenomenon is not peculiar to the genital region normally contacted by the male during mating.