There is a large influx of water into the pupal testis ofM. configurataduring eupyrene spermiogenesis, and subsequently, an almost equally large efflux of water from the testis after all the spermatozoa within spermatocysts have become fully elongated. The temporal pattern, direction and magnitude of change in the taurine content of the testis are similar to the movement of testis water, indicating that testis hydration-dehydration and taurine influx-efflux are linked osmotically. An osmotic model for membrane translocation in me testis during spermatogenesis is suggested in which low molecular weight precursors such as amino acids in the haemolymph that are required for spermiogenesis cross the testis wall in response to the osmotic gradient generated by testis hydration. After the precursors have been fixed in macromolecular structures of the elongated spermatozoa, waste products and other substances leave the testis with water during testis dehydration.