The aim of this study was to evaluate the distribution of mast cells in the ovary and uterus of rats during their oestrous cycle. Fourty female Wistar Albino rats, 10-12 weeks old, were used. Ovary and uterus tissues were fixed in Mota's fixative for 48 It and embedded in paraffin. Sections of 6 mu m thickness were cut and stained with toluidine blue (1% aqueous solution) and Alcian blue-safranin (pH: 1.0, buffered by using 0.1 N HCl. In the ovary, the mast cells were mainly present in the tunica albuginea, in the interstitial areas between follicles or corpora lutea and in the vicinity of blood vessels in the medulla. The number of mast cells in the ovarian medulla and cortex and the uterine endometrium and myometrium were highest in the oestrous. metaoestrous, oestrous and metaoestrous phases, respectively. The number of mast cells was higher in the ovarian medulla (7.4 +/- 0.52) and cortex (2.1 +/- 030) during oestrus rather than in other phases of the oestrous cycle (P < 0.05), with the highest number in the endometrium during the oestrous (6.8 +/- 0.45) and metaoestrous (6.2 +/- 0.56) and in the myometrium (7.1 +/- 0.63 for oestrous, 7.5 +/- 0.33 for metaoestrous) of uterus compared to other phases of the cycle (P < 0.05). Safranin-stained mast cells were numerous in all tissues during all stages of the oestrous cycle except the proestrous. Mast cells were safranin positive in the myometrium only during proestrous. It was concluded that some physiological changes could be responsible for the variation in the distribution of mast cells in the ovarian and uterine tissues of the rat during the oestrus cycle.