Parathyroid hormone (PTH) related peptide (PTHrP) is thought to influence the proliferation and differentiation of the epidermis and hair follicle. As a means of elucidating the biologic function of PTHrP on the hair follicle, a PTHrP analog PTH (7-34), which is a PTH/PTHrP receptor antagonist, was given intraperitoneally twice daily to C57 BL/6 mice at different stages of the hair cycle. PTH (7-34) induced 99 +/- 4.5% (mean +/- SEM) of resting telogen hair follicles into a proliferative (anagen) state, whereas 100% of the hair follicles in the control group remained in telogen. To determine whether this peptide influenced the progression of the hair follicles from anagen to catagen (hair follicle maturation and regression), groups of mice that were either spontaneously in or induced to anagen received either PTH (7-34) or placebo. Morphometric analysis of the hair follicles from the middle back region of the spontaneous anagen mice that received PTH (7-34) revealed that 19 +/- 4% (mean +/- SEM) of the follicles were in anagen VI, whereas none (0%) were in anagen in the control group. Similarly, in induced anagen mice treated with PTH (7-34), 22.3 +/- 1.4 (mean +/- SEM) of the follicles were in anagen VI compared to only 1.3 +/- 0.7% in the control mice. Together these observations suggest that PTHrP is a hair follicle morphogen that may be a major factor responsible for controlling the hair cycle. These studies provide a new insight for development of PTHrP analogs for a wide variety of disorders related to disturbances of hair cycling.