It has been demonstrated by various workers in the past that glucocorticosteroids block hair growth. Using the mouse model for studying hair growth induction we reexamined the effect of topically applied steroids on hair growth to establish at what stage the steroid block acts. In accord with studies by others, we found that these steroids block hair growth at the point of anagen initiation, but that once the steroid applications are stopped, hair growth starts. Since steroid withdrawal alone did not induce hair growth, it is clear that these steroids do not block, either spontaneous or manipulated, hair growth induction, but they do block, the apparent next step, i.e., hair formation. Moreover, since hair growth could be induced even while the animals were being treated with the steroid, the induction step appears independent of the steroid block-These studies and those of others lead us to conclude that these steroids block the expression of hair-forming genes, but do not interfere with the signal(s) that initiates those genes. This system appears to be ideal for identifying the signals (perhaps, genes) responsible for initiating hair growth.