To evaluate the effect of milk replacer (MR) feeding rate on cortisol and metabolic responses to an ACTH challenge, 39 Holstein calves (BW = 40.8 ± 5.2 kg) were randomly assigned to 3 dietary treatments: control (CON, 0.57 kg/d of a 20% CP and 20% fat MR), intermediate (IL, 0.68 kg/d of a 26% CP and 17% fat MR [26:17]), or high (HL, 0.79 kg/d of 26:17) at 2 d of age. Calves were housed in polyethylene hutches and managed similarly throughout the trial. Milk replacer was offered twice daily (12% solids) until d 42 when MR was fed once daily to decrease intake by 50%. Calves were weaned at d 49 and remained on trial until d 56. Plasma was collected at d 2, 5, 10, 14, 28, 42, 43, 45, 47, 49, 51, and 56 for analysis of cortisol. A subset of calves (n = 8/treatment) was subjected to an ACTH challenge at d 40. Plasma was collected at −30, −15, −5, 5, 10 15, 30, 45, 60, 90 and 120 min relative to ACTH infusion (0.125 IU/kg of BW) for analyses of cortisol, insulin and metabolites. Data were analyzed using the mixed procedure of SAS. During the experiment, the average temperature-humidity index was 75 inside and outside hutches and the average calf rectal temperature was 39.6 ± 0.4°C indicating consistent exposure to heat stress. Feeding rate of MR did not influence (P > 0.13) plasma cortisol concentrations during the experiment or the ACTH challenge. The ACTH infusion decreased insulin concentration and increased nonesterified fatty acid concentration (P < 0.01). After ACTH infusion, the plasma glucose concentration of HL calves was greater compared with CON and tended to be greater than IL (P = 0.08). Insulin concentrations were greater for HL compared with CON and IL (P = 0.03). Plasma nonesterified fatty acid concentrations of CON calves were greater than HL and tended to be greater than IL at 15 min and were greater than IL at 45 min following infusion (treatment × time, P = 0.02). Feeding increasing amounts of MR had no effect on plasma cortisol concentrations during the preweaning period and during an ACTH challenge; however, higher MR allowance may alter metabolism during acute heat stress.