This study was conducted to evaluate the effect of Magnolia Officinalis and 4-O-methylhonokiol, a main component of M. Officinalis, on the promotion of hair growth. When rat vibrissa follicles were treated with an ethanol extract of M. Officinalis and 4-O-methylhonokiol, the hair-fiber lengths of the vibrissa follicles increased significantly. In addition, after daily topical application of 4-O-methylhonokiol onto the back of C57BL/6 mice, anagen progression of the hair shaft was induced. Moreover, 4-O-methylhonokiol increased both the expression of proliferating cell nuclear antigen (PCNA) in the bulb region and proliferation of immortalized vibrissa dermal papilla cells. In order to determine the mechanism by which 4-O-methylhonokiol promotes hair growth, we examined the expression levels of transforming growth factor-beta1 (TGF-beta1) and TGF-beta2 which have been known to play an important role in anagen to catagen transition via the induction of keratinocyte apoptosis. When the vibrissa follicles in the anagen phase were treated with 4-O-methylhonokiol for 7 days, the expressions of TGF-beta1 in cells of outer root sheath (ORS) and epithelial strand, i.e., in the epithelial residue of the regressing hair bulb during catagen were found to be lower than those of the control follicles. Also, 4-O-methylhonokiol decreased the expression of TGF-beta2 in the bulb matrix region of the 7-day cultured follicles. These results suggest that 4-O-methylhonokiol has the potential to promote hair growth via down regulation of TGF-beta1 and TGF-beta2, as well as the proliferation of dermal papilla.