The osmolarity of the semen sample should be the next parameter checked. This may be done by means of an osmometer (such as Micro Osmometer Automatic, Type 13 Autocal, Roebling, Germany). The normal osmolarity range should be between 290 and 310m0sm (Pickett et al., 1976, 1989). Values outside this range cause water to diffuse out of or into the spermatozoon head and tail, down the osmotic pressure gradient across the semi-permeable plasma membrane. Hypotonicity will cause swelling and deformation of the cell, especially the tail, and may result in rupture of the plasma membrane. Hypertonicity causes dehydration, which adversely affects spermatozoon function. An osmolarity greater than 350 mOsm may occur in urospermia and values less than 200 mOsm can cause severe changes in spermatozoon morphology - typically swelling, bending or coiling of the tail (Varner and Schumacher, 1991). The extent of the damage will vary considerably, depending on the hydration staus of the horse (S. Revell, Wales, 1998, personal communication).

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