Since mammalian spermatozoa are generally considered to be less affected by hypertonic than by hypotonic solutions (see Mann, 1964) it was surprising to find that dialysis of ram semen for 6 hr in the continuous flow dialysis apparatus (CFDA) of Dott & Walton (1960) against a hypotonic diluent, did not affect the impedance change frequency (ICF) whereas a hypertonic diluent abolished the ICF within 2 hr (Quinn, White & Voglmayr, unpublished). The most obvious difference between the CFDA and diluting semen directly in tubes is that high molecular weight substances present in the seminal plasma remain within the dialysis sac and are not further diluted. To test whether the non-dialysable components of the seminal plasma were responsible for the effect, ram semen was diluted with 10 vol. of Krebs-Ringer buffered to pH 7·4 with 50 mm sodium