Aim: To investigate the spermatozoal production rate of the testis and the spermatozoal storage capacity of the epididymis in monkeys and rats. Methods: The number of the late spermatids (steps 13-14 in the monkey or steps 15-19 in the rat) per testis and the number of spermatozoa per epididymis were estimated in 6 normal adult monkeys (Macaca fascicularis) and 6 normal adult SD rats on 25 m-thick methacrylate-embedded sections using a contemporary unbiased and efficient stereological methodthe optical disector. The diameter and length of the efferent ductules and ductus epididymidis and the volume of the epididymal fluid in the tubules were also estimated. Results: The total number of the late spermatids per testis was 2902749 (million, means) in the monkey, or 17931 in the rat; the number of spermatozoa per epididymis was 32351835 in the monkey, or 24176 in the rat. Conclusion: A large number of spermatozoa was densely packed and stored in the ductus epididymidis; the epididymal transit time for spermatozoa was around 5 days in monkeys or 11 days in rats

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