Terminal follicular dynamics and ovulation rates (OR) were compared in different local breeds after introducing fecundity genes of different origin. Crossbred ewes which were carriers (F +) or non-carriers (+ +) of Booroola ((B)Fec) or Cambridge Penes ((C)Fec) were included: Cambridge x Cambridge (CC), Cambridge x Suffolk (CS), Cambridge x Texel (CT), Booroola x Texel (BT) and Booroola x German Mutton Merino (BGM). The numbers of small (diameter 2-3.5 mm), medium (diameter > 3.5-5.0 mm) and large (diameter > 5.0 mm) growing follicles, the maximum diameter before ovulation and the regression and artesia rates of ovarian follicles greater than or equal to 2 mm in diameter were studied laparoscopically and repeatedly during the last 5 days of an induced oestrous cycle. The ORs were determined one cycle before and two cycles after the repeated laparoscopy. (B)Fec and (C)Fec significantly enhanced the OR of all crossbreeds. Carriers of (B)Fec or (C)Fec did not have significantly different ORs due to any crossbreeding effect. The same observation was made for non-carriers of both Fec gene types. Whatever the crossbreed, the number of small, medium and large growing follicles were similar between carriers and non-carriers in spite of a higher number of ovulating follicles in carriers of both Fec gene types. The diameter of ovulatory follicles did not differ among crossbreds, or between carriers and non-carriers except in the BT (5.2 +/- 0.2 vs. 65 +/- 0.8 mm, respectively) and CC (6.6 +/- 0.2 vs. 5.6 +/- 0.3 mm) ewes. The higher OR in the presence of the Booroola gene was associated with a low atresia rate of large follicles in all crossbreeds (BT: 52 +/- 8\% (F +) vs. 61 +/- 7\% (+ +); BGM: 51 +/- 6\% vs. 75 +/- 5\%). The high OR of the carriers of the (C)Fec gene seemed to be associated with a lower number of large growing follicles with a lower(P < 0.05) atresia rate as compared with Booroola crossbreeds. In conclusion, follicular features were similar between purebred Cambridge and its crossbred CS and CT. In ewes carrying the (B)Fec or (C)Fec gene, the reduction in follicular atresia seemed to be one of the main follicular features implicated in the higher OR. (C) 2000 Elsevier Science B.V. All rights reserved.