Doležel R., Chambert C., Musilová D., Čech S., Páleník T. (2017): Induction of accessory corpus luteum by gonadorelin in relation to the time of treatment and the follicle size in inseminated cows. Czech J. Anim. Sci., 62,[195][196][197][198][199][200] This study evaluated the occurrence of two corpora lutea (CL) and the pregnancy rate in cows treated with gonadorelin on days 5, 6 or 7 after insemination in relation to the day of treatment and the size of the largest follicle on the ovaries at the time of treatment. Cows bearing one CL on days 5 (n = 40), 6 (n = 48), and 7 (n = 47) after insemination with defined size of the largest follicle on the ovaries (small ≤ 9 mm, n = 34; medium 10-14 mm, n = 59; large 15-20 mm, n = 42) were included in the study. Gonadorelin was administered after examination (day 0) and ultrasonographic examination was repeated on days 14, 28, and around day 90. The occurrence of 2 CL vs 1 CL was higher in pregnant cows on day 14 as well as on day 28 (86 vs 14% and 82.5 vs 14%, n = 57). Pregnancy rates on day 28 and around day 90 were higher in cows bearing 2 CL (n = 57) vs cows bearing 1 CL (n = 54) (82.5 vs 18.5% and 79.0 vs 18.5%, P < 0.001). The occurrence of 2 CL on day 28 was higher in cows treated on day 5 compared to cows treated on day 6 or 7 after insemination (60.0 vs 33.3 or 36.2%, P < 0.05). No significant differences in the number of CL and pregnancy rate were found in relation to follicle size at the time of treatment. Our results showed the higher efficiency of accessory CL induction by gonadorelin administration in cows treated on day 5 vs those treated on day 6 or 7 post insemination, however efficiency of the treatment was not influenced by the size of the largest follicle on the ovaries.