Several possible mechanisms to account for hormone-induced enlargement of receptive field areas of individual mechanoreceptive trigeminal neurons of the rat were investigated. Mechanoreceptor sensitivity was estimated by receptive field thresholds and showed no consistent change following systemic estrogen treatment. Alteration of the viscoelastic properties of skin was suggested as 10 days of estrogen treatment caused an acute epidermal hyperplasia (38.7%), but force-displacement measurements revealed no significant change in skin distensibility. Interestingly, both chemical (6-hydroxydopamine) and surgical sympathectomy mimicked the effect of estrogen on receptive field areas by promoting dramatic enlargement of individual neuronal fields without decreasing receptive field force thresholds. Among skin samples from estrogen, 6-hydroxydopamine and surgical sympathectomized animals, only 6-hydroxydopamine treatment caused any significant alteration in skin norepinephrine content. These results strongly suggest an indirect catecholamine involvement in estrogen-induced enlargement of receptive field areas.