The swimming rate of bull spermatozoa diluted in various media was estimated by the stop-watch technique. At a dilution of 20,000 spermatozoa per [mu]l in physiological saline solution, the mean of the mean swimming rates of spermatozoa from twenty-seven ejaculates from seventeen bulls was 155[center dot]9 [plus or minus] 5[center dot]0 [mu]/sec (range of means 107[center dot]0 to 203[center dot]2 [mu]/sec). The rate fell progressively with time during incubation at 37[degree]C. The rates at higher dilutions were progressively lower after only 5 min. incubation, and the greater the dilution the less well was the rate maintained, so that the differences between different dilutions increased with time. The percentage motile followed the same pattern as the swimming rate, fewest non-motile being present at the highest cell concentration. Maintenance of rate was slightly better than in saline solution and the dilution effect was diminished, but not abolished, if semen was diluted with seminal fluid (from the same ejaculate),"sperm extract" or Baker''s medium. The rates were slightly higher in the last two media than in saline solution, but in seminal fluid the rate was only about two-thirds that in saline solution. Spermatozoa swam more slowly in mucus from the uterine cervix of the cow than in any other medium tested. The mean of the mean swimming rates in forty-nine experiments with sixteen ejaculates and samples of mucus from ten cows was 62[center dot]7 [plus or minus] 1[plus or minus]7 [mu]/sec. The rate could only be related to dilution by counting the spermatozoa present in the mucus at the end of the experiment, and dividing the experiments into categories accordingly. Although the final concentration of spermatozoa ranged from 28 to 115,000/[mu]l, no dependence of swimming rate on concentration could be detected. At all dilutions in mucus, maintenance of rate was about equivalent to that for a 5000/[mu]l dilution in saline solution. About one-half of the spermatozoa which had swum into the mucus were non-motile at the end of 1 hr, at all dilutions, though of those which were initially alive in saline solution at 5000 dilution, only 41% died during the 1st hour, with correspondingly fewer dying at higher cell concentrations. The possibility that the dilution effect on swimming rate might be absent in mucus was also tested by measuring the rates at which spermatozoa swim through a given microscope field of mucus from the moment the first one appeared until a swarm of several hundred had invaded it several minutes later. There was no difference in rate. The impossibility of carrying out an exactly comparable experiment in saline solution was discussed. If spermatozoa were allowed to swim through a strip of mucus into saline solution, the mean rate and frequency distribution of rates in this saline solution resembled those of controls directly diluted in saline solution (to a concentration of 10,000/ [mu]l) much more than they resembled the figures for swimming rates in mucus. It was concluded that mucus does not have a "selecting" effect on spermatozoa with respect to swimming rate.