The purpose of this study was to compare the population of preovulatory follicles in two INRA strains of rabbits of different natural ovulation rates. The two strains chosen were the Californian (A1066) with an ovulation rate of 13 and the New Zealand (A1077) with an ovulation rate of 11. Nulliparous, primiparous and multiparous does were followed during the four seasons. Eight hours after mating the right ovaries of 154 of these animals were collected. The following data were recorded : the number of follicles with an area equal to or more than 5.10(5) micron 2 (diameter : 0.8 mm), the area of that section of the follicle containing the oocyte, and the presence of pyknotic cells with granulosa. The higher ovulation rate of Californian does was due to a significantly higher number of healthy follicles. The mean size of these follicles, their area (between 5.10(5) micron 2 and 3.10(6) micron 2) distribution and rate of atresia were similar in the two strains. Environmental factors led to changes in the number and size of healthy follicles which were responsible for the number of ovulations, while the rate of atresia remained constant. The seasonal effect was most marked in November by a definite reduction in follicle size, particularly in multiparous does. Follicle number and size increased with parity number; this seemed to be due to an acceleration of the growth rate and to the increasing number of large-sized follicles. As there were no interactions between genotype and environmental factors, it can be concluded that the latter modified follicular growth parameters in the same way in both strains and that these strains can be identified in all cases on the basis of their ovarian characteristics. About 1/3 of the does exhibited almost exclusively healthy follicles, while in the remaining 2/3 both healthy and atretic follicles were seen. These proportions were the same in both female genotypes. Those possessing only healthy follicles also had the largest ones. It is suggested that atresia could be a regulatory factor of ovulation rate along with the preovulatory level of FSH. 23% of the does accepting the buck showed no oocytes in meiosis, even though their follicle size exceeded 5.10(5) micron 2. Th mean number of these follicles was the same as that of does with oocytes in meiosis, but they were half as large. It is suggested that the follicle oestrogen secretion of these does was not high enough to induce ovulatory release of LH at mating.