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ABSTRACT. Oliveira J.P., Mello D., Silva O.R., Agostini C.R.O., Jacob J.C.F. & Mello M.R.B. [Assessment of vascular perfusion and cholesterol and estrogen levels in preovulatory follicles of mares in conditions of thermal discomfort.] Avaliação da perfusão vascular e do nível de colesterol e estrógeno em folículos préovulatórios de éguas em condições de desconforto térmico. Revista Brasileira de Medicina Veterinária, 38(Supl.2):67-74, 2016. Departamento de Reprodução e Avaliação Animal, Instituto de Zootecnia, Universidade Federal Rural do Rio de Janeiro, BR 465 Km 7, Seropédica, RJ 23890-000, Brasil. Email: email@example.com The aim of this study was to evaluate the pattern of blood perfusion of dominant follicles after ovulation induction with hCG by ultrasound Color Doppler, and cholesterol and estrogen levels in preovulatory follicles of mares submitted or not to thermal discomfort. Therefore, estrous cycles of 15 mares were randomly distributed between comfort (Group I) and thermal discomfort (Group II) groups and monitored by transrectal ultrasonography periodically until the largest follicle reached at least 32mm of diameter. At this time, the mares received 1000IU of Chorulon® (hCG), and had the preovulatory follicle accompanied by Doppler ultrasound every 6 hours up to 24 hours, at which time the aspirations were taken from the follicular fluid of cholesterol and estrogen dosing. The follicular vascular perfusion was estimated subjective basis, taking into account the percentage of the circumference of the follicular wall with Color Doppler signals. Aspirated follicular fluid was recovered and centrifuged at 1000G for 15 minutes, and the supernatant recovered and stored in cryovials at -20°C until the estrogen dosage and cholesterol were performed. To characterize the thermal environment, the temperature and humidity index test (THI), described by Hansen (2005), was used. In order to characterize the adaptation of animals to the thermal environment, the heat tolerance coefficient (HTC) and the adaptability coefficient (AC) were used. The average THI found for the analyzed period (March and April / 2014) was 58.33, which does not characterize stressful environment for the animals. After bioclimatic analysis found HTC average values of Group I and Group II 95.47, 87.14, these averages were statistically different (p<0.05), indicating a greater tolerance of Group I when compared to Group II. The AC average found for the group I was 3.84 and the Group II 4.29, values that differed statistically (p<0.05), showing greater adaptability in Group I compared to Group II. The average values of vascular perfusion of preovulatory follicles were: Group I: H0=32.5%; H6=43.75%; H12=41.85%; H18=33.75%; H24=42.5%; Group II: H0=24.85%; H6=41.42%; H12=48.57%; H18=38.57%; H24=47.14%. There were no statistically significant differences (p>0.05) between the percentages of follicular vascular perfusion between the groups in the analyzed moments. The mean values of cholesterol and estrogen for Groups I and II were respectively 51.62mg/ml and 46.14mg/ ml and 325739.64pg/dL and 316381.05pg/dL. There were no statistically significant differences (p>0.05) between groups. These results demonstrate that environments that deprive the shadow animals are likely to generate thermal discomfort, even if the indices do not point stressful environment for the animal. However, this discomfort is not enough to harm steroidogenesis or percentage follicular vascularization in 24 hours.