The extra-gonadal sperm reserve (the sperm reserve in the vasa deferentia and the epididymides) was depleted in six donors by an average of 2·4 emissions per day during a 10-day depletion period. Daily sperm output (dso) was then estimated from the examination of one specimen a day for 24 days. During the dso period, specimen volume averaged 59% and sperm concentration per specimen 27% of that in the control or pre-depletion period. Sperm output per specimen and per week did not return to the levels of the pre-depletion period during a 156-day period (equal to three complete 52-day cycles of spermatogenesis and movement of spermatozoa through the duct system) after the completion of dso. At a frequency of fewer than five emissions per week, the greatest single influence on sperm numbers in the specimen is the size of the extra-gonadal sperm reserve. Daily sperm output, after the depletion of the extra-gonadal sperm reserve, may be used to estimate daily sperm production (dsp) by the testes. Changes in dso may be used to measure treatment effects on spermatogenesis. A practical experimental design is proposed for the analysis of factors and treatments affecting dsp in man.