The present paper is to report the results of some microscopical observations on naturally and artificially induced ovulation in mature and immature rats, with special reference to the number, stage and some other morphological features of ovulated ova.<br>Based on the data obtained from the observations of the whole 288 adult females under study, the duration of each stage of the oestrus was estimated in average as follows: 56.5 hours for the dioestrus stage, 13.5 hours for the preoestrus stage, 25.3 hours for the oestrus stage, and 13.3 hours for the metooestrus stage. It is then evident that the whole duration necessary for an oestrus cycle is 108.7 hours (approximately 4.5 days) on an average (cf. Table 1). The ovulation seems to be taken place during the period from 12 to 20 hours in the oestrus stage.<br>The number of eggs discharged at natural ovulation shows a variation from 8 to 10 in the majority of cases, being 8.8 on an average (cf. Table 2). These eggs are 75 to 81 micra in diameter, and possess the second polar spindle at metaphase.<br>Through the injection of the gonadotropic hormone “Hypohorin” (Teikoku Hormone MFG. Co.), ovulation was artificially induced in mature females, though it was necessary to determine the effective time of hormone application in the oestrus cycle. In an extreme case, a mature rat discharged 40 eggs, 31 from the left ovary and 9 from the right ovary, following the application of 30 rabbit units Hypohorin which was used during the period from the dioestrus stage to the preoestrus stage. A rat which recieved injection at the same dosage in the period from the preoestrus stage to the oestrus stage discharged 37 eggs (26 left: 11 right), while the other laid 19 eggs (9 left: 10 right) by hormone application during the period from the dioestrus stage to the preoestrus stage. The data together with those from some other rats are given in Table 3.<br>In every case, the ova discharged by hormone injection show cytologically normal maturation: they carry the second maturation spindle which has persisted in the metaphase stage, and have a diameter ranging from 75 to 81 micra.<br>The artificial ovulation was successfully induced in immature rats 15 to 20 hours after the injection of Hypohorin at a dose of 2-10 rabbit units. The data are shown in Table 4. The eggs discharged by this treatment are nearly normal both in size and in some other morphological aspects, as compared with those from adult rats.<br>The evidence presented indicates that the ova which were discharged by gonadotropic hormone are essentially similar, not only in nature but also in morphology, to those obtained in natural ovulation.