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Abstract. The diameter of the preovulatory follicle (POF) and its effects on subsequent corpus luteum (CL) size and conception were studied in 38 lactating indigenous cycling buffaloes in the Mymensingh district of Bangladesh. Body condition score (BCS) at estrus was estimated for the buffaloes. The buffaloes were synchronized with two injections of a synthetic analogue of PGF2 α administered 11 days apart. Transrectal ultrasonography was carried out at estrus and on days 5, 9, 12 and 16 post ovulation to determine the POF and successive CL size. Pregnancy was confirmed by ultrasound examination on day 40-45 post ovulation. Twenty one (55.3%) buffaloes were diagnosed as pregnant. The conception rates of thin (BCS ≤2.0), good (BCS 2.5-3.5) and fat (BCS >3.5) buffaloes were 7.7, 88.2 and 62.5% (χ 2 = 19.54; P<0.05), respectively. The mean diameter of the POF at estrus was larger (P<0.01) in buffaloes that ultimately were diagnosed as pregnant compared with their nonpregnant counterparts (13.7 ± 0.3 vs. 11.2 ± 0.5 mm, respectively). The conception rates of buffaloes having small (9 to ≤ 12 mm), medium (>12 to ≤14 mm) and large (>14 to 16 mm) POFs at estrus were 9.1, 70.0 and 85.7% (χ 2 = 13.87, P<0.01), respectively. On day 5 post ovulation, CL size was positively correlated (CL: r=.74, P<0.01) with POF diameter. Retrospective analysis revealed that on day 5 post ovulation, the pregnant buffaloes had higher (P<0.01) post ovulation CL sizes than their nonpregnant counterparts (15.6 vs. 11.8 mm). Similarly, on day 9 post ovulation, the difference in CL size (14.3 vs. 13.6 mm) between pregnant and nonpregnant buffaloes was significant (P<0.05). In conclusion, the diameter of the POF in buffaloes has a positive impact on the size of the post ovulation CL and conception. Key words: Corpus luteum (CL), Preovulatory follicle (POF), Water buffalo (J. Reprod. Dev. 58: [219][220][221][222] 2012) T he preovulatory follicle (POF) is the prime structure behind the endocrinology of estrus in buffaloes that produces substantial amounts of estradiol. A positive correlation exists between the diameter of the POF and plasma estradiol concentration and the subsequent pregnancy rate in cattle [1,2]. It is speculated that the POF diameter is important for the subsequent development of the corpus luteum (CL) and hence conception in buffaloes. A larger POF may generate a larger CL that secretes more progesterone and thereby have a positive effect on pregnancy recognition and pregnancy rates in cattle [3,4]. On the other hand, others have reported absence of correlation [5] or negative correlation [6] between POF diameter and pregnancy outcome. For successful pregnancy, the preovulatory follicle and subsequent luteal size may be important in the indigenous buffaloes. But reports on such work in indigenous buffalo of Bangladesh are not available in the literature. So, it is important to investigate the relationship of POF size and subsequent CL size and pregnancy rate. Use of ultrasonic imaging of ovarian follicular and luteal developme...