In mammals hair growth is cyclical; however, the factors that regulate the hair growth cycle are still poorly understood. The recent development of methods for culturing hair follicles in vitro has proved an important tool to investigate many aspects of the regulation of hair follicle growth. At present, however, these models are based on the culture of anagen hair follicles and have only partially been used to address the cyclical nature of hair growth. In this study we have made use of the fact that in rodents the hair growth cycle is synchronized, well characterized, and relatively short. We have isolated vibrissa follicles from 12 d old rats and confirmed by histology that these follicles are in the anagen stage of their first hair growth cycle. We have then maintained these follicles in vitro, on Gelfoam supports, for up to 23 d (35 d of age) and compared their histology with in vivo follicles from equivalent age littermates. We observed that 12 d old follicles maintained in vitro for up to 23 d show changes in morphology that suggest that cultured rat vibrissa follicles retain cyclical activity in vitro. Cyclical changes in hair follicle morphology were only seen in follicles maintained on gelfoam supports and moreover, hair follicle size appears to be a key feature in determining the ability of the follicle to cycle in vitro. All follicles that showed cyclical changes in vitro, however, appeared to remain blocked in pro-anagen. These data suggest that the vibrissa follicle is a in vitro good model system with which to investigate hair cycle control. J Invest Dermatol 115:1152-1155 2000