The concentration of sialic acid was determined in the reproductive organs, pituitary gland and urine of male and female langur monkeys. In the male, sialic acid concentration was lowest in the testes and highest in the epididymides. Both caput and cauda epididymides contained almost the same concentration. Castration and subsequent androgen administration did not modify the concentration of sialic acid by comparison with intact animals, suggesting that sialic acid is not androgen-dependent in the langur. There was no significant difference in sialic acid concentration in the uterus during the proliferative and luteal phases of the menstrual cycle. After ovariectomy, there was a decrease in the concentration of sialic acid in the uterus, cervix and vagina while oestrogen or oestrogen plus progesterone administration increased its concentration significantly, but not to the control level. However, the difference in concentration between oestrogen-treated and oestrogen plus progesterone-treated monkeys was not significant. There was a peak of sialic acid excretion in the urine on day 8 of the normal 22-day menstrual cycle, i.e. a day before the probable day of ovulation. In pregnant monkeys siliac acid excretion increased significantly 2 days before delivery compared with the values recorded 7 to 3 days before parturition. The part played by sialic acid during the menstrual cycle and pregnancy in the female is not clear.