Vacas da raça Holandesas em lactação (n=158) aos 213±112 dias de lactação e produção de 26±9kg leite/dia, foram aleatoriamente distribuídas em três grupos: controle (GC, n=52, salina); GnRH (GG, n=55, 100mcg de gonadorelina); e hCG (GH, n=51, 2500UI de hCG) aplicado no dia 5 após a inseminação artificial (IA). A temperatura retal foi verificada no momento da IA, e as amostras de sangue coletadas nos dias 5, 7 e 12 após a IA. A concepção foi determinada entre os dias 42 e 49 após IA. As concentrações séricas de progesterona (P4 - ng/ml, média±EPM) para GC, GG, e GH foram, respectivamente: no dia 5: 2,7±0,4, 2,5±0,4 e 3,2±0,4; no dia 7: 4,8±0,4, 4,2±0,4 e 5,7±0,5; e no dia 12 após a IA: 5,2±0,4, 6,9±0,4 e 8,5±0,5. O aumento proporcional na concentração sérica de P4 entre os dias 5 e 7 após IA (GC: 178%, GG: 168%, e GH: 178%) sugere que os tratamentos não induziram efeito luteotrópico no corpo lúteo (CL) existente. O aumento na P4 sérica entre os dias 7 e 12 nos animais tratados com GnRH ou hCG (GG: 164% e GH: 149%, P<0,01) em relação aos animais controle (GC: 18%, P=0,31), sugere a indução de novo CL. Os tratamentos com GnRH ou hCG aumentaram as taxas de concepção nas vacas com temperatura retal abaixo de 39,7°C (GC: 10,1%, n=26; GG: 36,8%, n=27 e GH: 32,8%, n=21), mas não em vacas com temperatura retal acima de 39,7°C (15,2% n=26; 17,8%, n=28 e 24,4%, n=30). Os resultados sugerem que a alta temperatura corporal pode mascarar os efeitos positivos do tratamento com GnRH ou hCG no dia 5 após a IA, na concepção.Lactating Holstein cows (n=158), at 213±112 days in milking and averaging 26±9kg of milk per day, were randomly assigned to one of three treatment groups: control (CG, n=52, saline), GnRH (GG, n=55, 100g gonadorelin), and hCG (HG, n=51, 2500IU) given five days after artificial insemination (AI). Rectal temperature was taken at the moment of AI and blood samples were collected five, seven, and 12 days after AI. Pregnancy was determined between 42 and 49 days after AI. Concentration of progesterone (P4) in serum (ng/ml, mean±SE) for CG, GG, and HG were, respectively, 2.7±0.4, 2.5±0.4, and 3.2±0.5 on day 5; 4.8±0.4, 4.2±0.4, and 5.7±0.5 on day 7; and 5.2±0.4, 6.9±0.4, and 8.5±0.5 on day 12 after AI. P4 concentration had proportional increase in serum between days 5 and 7 after AI (CG: 178%, GG: 168%, and HG: 178%), suggesting that the treatments did not induce a luteotropic effect on the existing corpus luteum (CL). Concentrations of P4 increased between days 7 and 12 in cows treated with GnRH and hCG (GG: 164%, and HG: 149%, P<0.01); but not in control cows (GC: 18%, P=0.31), suggesting that a new CL was formed. Treatments with GnRH or hCG increased conception rates in cows with rectal temperature below 39.7°C (CG: 10.1%, n=26, GG: 36.8%, n=27; and HG: 32.8%, n=21), but not in cows with rectal temperature above 39.7°C (CG: 15.2%, n=26; GG: 17.8%, n=28; and HG: 24.4%, n=30). These data suggest that high body temperature masked the positive impact of treatment with GnRH or hCG on day 5 after AI on conception.