Heat shock proteins of 72 and 90 kDa (HSP70, HSP90) are thought to be involved in the functional modulation of sex steroid receptors. To examine the expression and transcriptional regulation of HSP70 and HSP90 in both the endometrium and myometrium, messenger ribonucleic acid (mRNA) and protein level expression of these HSPs during various phases of the menstrual cycle were analyzed by Northern blotting and immunohistochemistry. In the endometrium, HSP70 mRNA levels increased during the secretory phase compared with levels during the proliferative phase; this is consistent with the stronger immunostaining of HSP70 in glandular cells in the secretory phase. In the myometrium, however, mRNA and protein expression of HSP70 were higher in the proliferative phase than in the secretory phase. This indicates that the regulatory mechanism of HSP70 expression during the menstrual cycle differs between the endometrial glandular cells and myometrial smooth muscle cells and may be correlated with the period of sex steroid-related functions of the respective cells. Expression of the HSP90 mRNA and protein in the myometrium was higher in the proliferative phase than in the secretory phase. In the endometrium, HSP90 immunostaining in the glandular cells was also stronger in the proliferative phase, although expression of HSP90 mRNA was not significantly different between the tissues of the two phases.