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In addition, the onset of tubular fluid production at 20-35 days coincides with a prepubertal rise in FSH. Based on these observations and our findings of a dilated Sertoli cell cytoplasm in FORKO mice, it seems likely that FSH-R regulates water balance in the Sertoli cells. Recent findings reported by Haywood et al. using the hpg mouse model lend support to this hypothesis. These mice lack gonadotropins, and constitutively express a mutated FSH-R, showing increased levels of tubular fluid secretion resulting from overactive FSH-R signaling. Recently, the expression and immunolocalization of several water channel proteins, aquaporins (AQPs), have been described in the testis of rats. AQPs are essential for regulating water homeostasis and for providing sustained and rapid movement of fluid across a tightly sealed epithelium with minimal activation costs. There are at least 11 different members of the AQp family, with several containing cAMP motifs and CREB binding regions that regulate their transcription. A systematic effort to localize AQPs in the testis of these mice would be required to fully understand hormonal/receptor regulation of these channels.