We recently reported that the number of hypothalamic tanycytes expressing pro-opiomelanocortin (Pomc) is highly variable among brains of adult rats. While its cause and significance remain unknown， identifying other variably expressed genes in tanycytes may help understand this curious phenomenon. In this in situ hybridization study， we report that the Prss56 gene， which encodes a trypsin-like serine protease and is expressed in neural stem/progenitor cells， shows a similarly variable mRNA expression in tanycytes of adult rats and correlates inversely with tanycyte Pomc mRNA. Prss56 was expressed in α1， β1， subsets of α2， and some median eminence γ tanycytes， but virtually absent from β2 tanycytes. Prss56 was also expressed in vimentin positive tanycyte-like cells in the parenchyma of the ventromedial and arcuate nuclei， and in thyrotropin beta subunit-expressing cells of the pars tuberalis of the pituitary. In contrast to adults， Prss56 expression was uniformly high in tanycytes in adolescent rats. In mice， Prss56-expressing tanycytes and parenchymal cells were also observed but fewer in number and without significant variations. The results identify Prss56 as a second gene that is expressed variably in tanycytes of adult rats. We propose that the variable， inversely correlating expression of Prss56 and Pomc reflect periodically oscillating gene expression in tanycytes rather than stable expression levels that vary between individual rats. A possible functional link between Prss56 and POMC， and Prss56 as a potential marker for migrating tanycytes are discussed.