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The mechanism by which an intra-uterine device in one horn of the rat uterus prevents implantation in that horn is not known. It has been suggested that histamine has a role to play in the implantation of the fertilized ovum (Shelesnyak, 1952). Shelesnyak, Kraicer & Zeilmaker (1963) have indicated that on the latter part of the 3rd day after successful mating in rats there is an oestrogenic upsurge which triggers off histamine release and this is essential for successful implantation. It has been known for a long time that there is a correlation between the histamine content of a tissue and the mast cell count (Riley & West, 1953). Mast cells have been demonstrated in the uterine tissue of golden hamsters (Harvey, 1964), guinea-pigs (Iversen, 1962), ferrets (Buchanan, 1966) and cows (Likar & Likar, 1964). Changes in the mast cell count at different phases of the oestrous cycle were observed in cows, hamsters and ferrets by the above mentioned workers who found an increase in the mast cell count in the oestrous phase. In view of the possible role of mast cells and histamine in normal implantation in the rat it was thought that a study of the mast cell distribution in the normal rat uterus and also in the uterus with an intra-uterine device in one horn would be worthwhile. It may give an indication as to whether the intra-uterine device exerts its anti-implantation effect by altering the mast cell content of the uterus. The following studies were carried out: (i) In ten female rats, five at the oestrous and five at the di-oestrous phase of the cycle, the mast cell distribution in the upper, middle and lower third of each horn of the uterus was counted by studying the mast cells present in each transverse section. Six slides were counted for every portion. Frozen transverse sections 45 µ thick, as indicated by Mota, Beraldo, Ferri & Junqueira ( 1956), were cut and identified by the characteristic metachromatic staining, (ii) In the second study, a small piece of Margulies spiral, 1 to 2 mm in length, was introduced into one horn of the rat uterus in nineteen animals during laparotomy under ether anaesthesia. These animals were killed 7 to 10 days after introduction of the device, ten at the oestrous and nine at the di-oestrous phase of the cycle and the mast cells stained and counted as described above in both the control horn and the horn with the device which was observed when the dissection was done, (iii) In

THE EFFECT OF AN INTRA-UTERINE PLASTIC DEVICE ON THE MAST CELL COUNT IN THE RAT UTERUS