At spawning the spermatozoa of freshwater fish are exposed to an environment of decreased osmotic pressure relative to the seminal plasma. This environmental shock triggers a transient motile phase assuming specific molecular changes. Flow cytometric investigations revealed the permeabilization of a fraction of the sperm cells as a result of the hypo-osmotic shock. The size of the permeabilized subpopulation was continuously growing for 4-20 min after which time it plateaued as it was monitored by the propidium iodide DNA staining technique. Plateau values were inversely correlated with ionic strength. Permeability changes in the spermatozoa during the response to dilution in low ionic strength solutions were accompanied by a decrease in the small angle forward scattered light signal (FALS). The reduced scattering intensity is explained by structural changes in the cytoplasmic membrane of the cells. Permeabilization and membrane structural changes are separated in time. The independent nature of these two processes are further supported by the observation that structural changes are reversible and can be reverted either by an increase in the extracellular Ca 2 + or restoring the physiological ionic strength. A possible explanation for the observed changes in FALS signal after hypo-osmotic shock is the reorganization of the lipid domain structure of the plasma membrane of the cells.This work was partially supported by OTKA grants 1463, T6184 and F5420.