Insectivora is the third largest mammalian order, composed of a number of unusual species considered to be the most primitive of the modern eutherian mammals. Yet, little is known about the members of this evolutionarily unique and important group. The musk shrew (Suncus murinus) is a convenient and practical laboratory animal. The study of the reproductive biology of this species will yield needed comparative data. Moreover, the little information that we have collected on this species suggests that several unusual characteristics make this animal a worthwhile and novel model for endocrine research. Most of the current and past research on this species has focused on the endocrinology of the female musk shrew. Unlike conventional small mammal models, the female musk shrew has no spontaneous ovarian cycle. The ovaries of the adult, unmated female do not undergo spontaneous follicular development. At the time of mating only small, relatively immature follicles are present. As a consequence, ovarian hormone production is not cyclic. Thus, traditional hormone target tissues such as the vaginal epithelium and the uterus do not exhibit cycles in cell proliferation. Sexual behavior is likewise demonstrated in an acyclic manner. Virgin females become sexually receptive within minutes after their first exposure to a male, and nonpregnant females are virtually always sexually receptive. Sexual receptivity in the virgin female musk shrew occurs in the face of relatively low plasma estradiol levels and higher androgen levels, and does not appear to be mediated via ovarian estradiol. Instead, recent work suggests that close to physiological doses of testosterone can restore sexual behavior in ovariectomized musk shrews.(ABSTRACT TRUNCATED AT 250 WORDS)