SUMMARY The content and concentration of sialic acid in the testis, caput epididy-midis, cauda epididymidis and ductus deferens show changes during the transition from the prepuberal to the puberal (mature) state in the rat. The peak of sialic acid is attained in the caput epididymidis and cauda epididy-midis and ductus deferens between days 40 and 45 prior to the entry of sperms into these regions. Following the entry of sperms there is a sharp decline in sialic acid till day 60 in the caput epididymidis and ductus deferens while in the cauda epididymidis the decline in sialic acid is only transient between days 45 and 50 and is followed by sharp increase at day 60. Efferent duct ligation caused a marked decrease in content of sialic acid in the caput epididymidis only. The significance of these changes is discussed in relation to the maturation of spermatozoa.