Overwintering Rana pipiens were studied to clarify physiological control of the nuptial pad, a digital specialization of male frogs that is used in mating. Testosterone cypionate implants stimulated nuptial pad growth in both castrated and sham-operated males relative to placebo-treated controls. Androgen-induced changes in the nuptial pad simulated seasonal recrudescence, and included significant increases in thickness of both the epidermis and dermis, formation of keratinized epidermal papillae, hypertrophy of the gland epithelium, enlargement of the gland lumina, and cytoplasmic accumulation of secretory product. Castration did not influence the response to testosterone, nor did it markedly affect nuptial pad morphology, probably because the pads were in maximal regression. These findings, in conjunction with previous work, implicate testicular androgens in seasonal cyclicity of the nuptial pad.