The phenomenon of hormone induced enlargement of receptive field area in mechanoreceptive neurons was investigated with respect to time course, hormone, sex and modality specificity. In acute recording experiments, threshold receptive field boundaries of single trigeminal ganglion neurons innervating the facial region of rats were determined for several different treatment groups. Among ovariectomized females injected with estradiol benzoate (EB) for 2, 5 or 10 days, receptive field areas were significantly increased after only 2 days. Enlargement of field areas appeared to be maximal after 5 days of EB. Lordosis behavior was observed after 10 days of EB but not after 2 or 5 days, suggesting that receptive field enlargement occurs independently of hormone induced lordosis behavior. In normal, intact females, receptive field area varied significantly with the estrous cycle (large during estrus, small during diestrus) indicating that this phenomenon is naturally occurring, not dependent upon the method of estrogen administration. Progesterone given for 10 days had no effect on receptive field area. Treatment with 2 days of EB followed by a single progesterone injection produced reliable lordosis but did not result in any further increase in receptive field area beyond the increase produced by 2 days of EB alone. Thus the synergism between estrogen and progesterone that is recognized for induction of lordosis is absent with respect to receptive field enlargement. The small increase in field area following 10 days of testosterone treatment could be accounted for by the fact that such animals, had blood serum estradiol levels comparable to animals given 2 or 5 days of EB, possibly the result of a testosterone to estrogen conversion. It was concluded that hormone induced receptive field enlargement appears specific for estrogen. Castrated male rats treated with 10 days of EB show the same increase in receptive field area as females, indicating that the phenomenon is not specific to females. Analysis of receptive field area as a function of receptive field type and adaptation rate showed that EB induced field area enlargement was specific to rapidly adapting mechanoreceptive neurons.