Spermatozoa were recovered from the isthmus of the rabbit oviduct at 4 and 11 h post coitum using several defined flushing media. The motility of spermatozoa in the isthmic flushings was subsequently analysed from video recordings. There was little sperm movement in the native isthmic fluid, but vigorous flagellar activity and hyperactivated movement were induced by flushing the isthmus with 0.25 M-sucrose, apparently an effect of dilution. Flushing with a complex culture medium resulted in similar stimulation of sperm movement. When pyruvate was present in the medium, hyperactivated flagellar bending was stimulated, whereas these movements were virtually absent when glucose alone was present. The stimulating effect of dilution was less pronounced when the flushing medium contained 50 mM-potassium. When the isthmus was flushed with media containing 50 mM-K+ and the K+ concentration was lowered to 5 mM during a washing procedure, large-amplitude flagellar movements were sequentially suppressed and restored. The restoration of large-amplitude movements was enhanced when pyruvate was present in the 5 mM-K+ washing medium. These results suggest that alterations in the concentration of both K+ and pyruvate may have a role in regulating the motility of rabbit spermatozoa in the oviducal isthmus, K+ being inhibitory and pyruvate stimulatory.