A 'flow dialysis' technique is described which enables spermatozoa to be maintained in an active state for prolonged periods of time, under conditions of constant temperature, gaseous partial pressure, pH, osmotic pressure and sperm density. The impedance-change frequency is used to assess sperm activity. The effects of centrifugation, dilution and washing of ram spermatozoa on their activity have been studied. Washing has been shown to produce a detrimental effect on sperm activity. This effect was most pronounced when a high degree of dilution was employed at each stage of the washing procedure. Washing produced a greater decrease in the activity due to aerobic endogenous respiration than in that due to anaerobic fructolysis. Other effects of washing included an increase in the proportion of 'eosinophilic' spermatozoa and an agglutination of motile spermatozoa. An attempt was made to assess the efficiency of the washing procedure by the use of fructose, [32P]phosphate and iodinated [131I]protein as indicators. The effect of dilution on spermatozoa depends on at least three factors: (i) the degree of dilution, (ii) the rate of dilution, and (iii) the composition of the diluent.