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Sixty-nine Holstein heifers were used to evaluate the effect of progesterone supplementation after timed artificial insemination (TAI) on conception rates of heifers under heat stress. Heifers were randomly assigned to 1 of 3 treatments: 1) control treatment (C), visual detection of estrus. Heifers detected in estrous were inseminated according to the AM-PM procedure (n = 23); 2) TAI (n = 24), using a CIDR device (insert containing progesterone) placed intravaginally on day -10 and removed on day -2 with PGF2α given on day -3. Heifers were given 0.5 mg of ECP at CIDR insertion and a second injection (0.5 mg ECP) was given on day -2 and 48 h after all the heifers were inseminated (day 0); and 3) TAI with progesterone supplementation (TAI+S; n = 22), consisting of TAI plus progesterone supplementation with a used CIDR from day 4 to 14 after insemination. The maximum temperature humidity index during the study averaged 86, while the minimum was 77. Blood samples were collected from all heifers on days 5, 9, and 14 post-insemination for quantification of progesterone, thyroxine, and triiodothyronine levels. Respiration rate (RR) was measured at 1700 hours and rectal temperature (RT) at 1800 hours, 3 times per week. The conception rate did not differ (P > 0.05) among treatments (C = 40; TAI = 34.6; TAI+S = 44.4%). The levels of progesterone were not increased (P > 0.05) by progesterone supplementation. Values for RR, RT, and thyroid hormones were similar among treatments (P > 0.05). TAI combined with progesterone supplementation did not improve the conception rate of Holstein heifers under heat stress. [ABSTRACT FROM AUTHOR]