A movement pattern known as hyperactivation has been observed among sperm recovered from the periovulatory oviduct of several species. In culture medium, hyperactivated sperm swim in a pattern that is far less progressive than that of freshly ejaculated sperm. In the oviduct, sperm encounter highly viscoelastic substances, such as mucus and the cumulus matrix. We have previously reported that hyperactivated hamster sperm become more progressive in vitro when the viscosity of medium is increased. In the present study, we tested the effect of increasing the viscosity and viscoelasticity of the medium on the swimming progressiveness of mouse sperm. Caudal epididymal sperm were incubated in a medium that produced hyperactivated motility in 60 min. Swimming velocities of sperm incubated for 60 min were compared with those of fresh sperm after addition of one of the following to culture medium: solutions of 1.8% methylcellulose (high viscosity), 1.8% long chain polyacrylamide (high viscoelasticity), or culture medium alone (low viscosity). In culture medium, hyperactivated sperm had significantly lower mean straight-line velocities than fresh sperm (p = 0.004); this difference disappeared in methylcellulose (p = 0.085) and was reversed in polyacrylamide (p = 0.004). This and other velocity measurements indicated that hyperactivated mouse sperm penetrate viscoelastic media more efficiently than fresh sperm and therefore may be more efficient at penetrating oviductal mucus and cumulus matrix in vivo.