The metabolism of dog spermatozoa in the presence of various ions has been studied. Of the ions tested, potassium had the greatest effect and at least doubled the respiration of washed cells. It did not, however, significantly change aerobic fructolysis. Due to interaction with other diluent components, magnesium ions had variable effects on respiration. Thus magnesium enhanced fructose oxidation in the presence of low levels of potassium but depressed respiration in the presence of low levels of calcium. Calcium improved respiration of washed spermatozoa and caused the accumulation of lactate without any concomitant increase in fructose utilization. Washing the spermatozoa free of their seminal plasma reduced the oxidation of substrates other than fructose. This effect was probably due to the removal of alternative substrates in the seminal plasma. The lactate present in dog semen only accounted for a small proportion of oxygen uptake and other alternative substrates are discussed.