Testicular and epididymal spermatozoal reserves were determined for 11 healthy men, a vasectomized man and 12 other men. For healthy men testis weight (19 testes) averaged 16.9 plus or minus 1.2 gm. and daily spermatozoal production was 4.25 times 10(6) per gm. (range equals 1.4 to 6.3 times 10(6) per gm.). Based on our total sample (23 patients) 67 per cent of the men 20 to 50 years old probably produce between 45 times 10(6) and 207 times 10(6) spermatozoa per day. Daily spermatozoal production was normal in 1 vasectomized man. Epididymal reserves of healthy men totaled 182 times 10(6) spermatozoa per epididymis, of which 26 per cent were in the caput, 23 per cent in the corpus and 52 per cent in the cauda. The interval between any previous ejaculation and death was unknown but we assumed that these values are typical for men copulating or masturbating every 1 to 7 days. Extragonadal spermatozoal reserves totaled approximately 440 times 10(6), of which less than 225 times 10(6) would be available for ejaculation in the ductuli deferentia and caudae epididymides. Transit time of spermatozoa through the caput, corpus and cauda epididymidis was estimated as 0.72, 0.71 and 1.76 days. In a vasectomized man extragonadal reserves totaled 7 times 10(9) spermatozoa or about 75 days of production by the testes. Thus, spermatozoal production in man is much less efficient than in other mammals, the extragonadal reserves of spermatozoa are small and maturation of spermatozoa in the caput plus corpus epididymidis occurs in less than 2 days. In terms of these quantitative characteristics the reproductive capacity of man is considerably less than that of a rhesus monkey.