Mature spermatozoa have a mechanism by which they can reduce cellular swelling caused by hypo-osmotic stress. The development of this ability during epididymal maturation in the bull was investigated. Caput and cauda sperm preparations were exposed to various osmotic stresses at 38°C and measurements of cell volume made by electronic cell sizing. (1) Epididymal spermatozoa recovered and incubated in a medium isotonic with caudal epididymal plasma (360 mOsm kg^–1) showed better viability and better volume regulatory ability than those incubated in a medium isotonic with seminal plasma (300 mOsm kg^–1) or in seminal plasma itself. (2) Preparations of both caput and cauda spermatozoa, isolated in a medium isotonic with caudal epididymal plasma, contained two volumetric subpopulations, unrelated to the presence or absence of attached cytoplasmic droplets. (3) The cell volume of both subpopulations of caput spermatozoa was always greater than that of the corresponding cauda spermatozoa subpopulations. (4) After exposure to hypotonic challenge, both caput and cauda spermatozoa were able to reduce their relative volumes, demonstrating that both immature and mature cells are able to express regulatory volume decrease under physiological conditions. (5) When spermatozoa were incubated in chloride- or sodium-free media, although two subpopulations remained present, the volume of the caput sperm populations decreased to that of their counterparts in cauda sperm preparations. It is concluded that immature caput spermatozoa are capable of regulating their volume in a similar fashion to mature cauda spermatozoa but are less able to control their isotonic volume, probably due to poorly controlled sodium and chloride ion transport.