Fifteen mature horses (mares, n = 6); geldings, n = 9) were used to assess the physiological responses of 24 h of transport in a commercial van under California summer conditions. The study was conducted on four consecutive days, and data were collected on d 1 and d 2 to obtain baseline values and to determine any diurnal variation in the individual measurements. Travel commenced on d 3 at 0800 for 24 h, with a total of 1,622 km traveled. Blood samples were collected at 0800, 1100, 1400, 2000, and 0200 each day. Horses were weighed and rectal temperatures recorded at 0800 each day and at 2000 each day except d 3. Body weight, rectal temperature, serum cortisol, serum lactate, and white blood cell (WBC) counts exhibited diurnal variation (P = .0001) on d 1 and d 2. Body weight immediately after unloading showed a 6% loss. At 24 h following transit, a 3% deficiency in body weight loss remained. The WBC counts showed a progressive increase with duration of travel and peaked at the termination of transport. Dehydration measures of hematocrit and total protein increased during transport and returned to baseline during the posttransport period. Serum concentrations of lactate, creatine kinase and aspartate aminotransferase increased during transport and in the early posttransit period, but returned to baseline values at the conclusion of the 24-h posttransport period. Glucose concentration increased with the initiation of transport and did not decrease to baseline concentration at the end of the 24-h posttransport period. Plasma cortisol and neutrophil:lymphocyte ratio increased with duration of transit and returned to baseline during the posttransport period. These data clearly showed physiological responses of horses undergoing 24 h of transport including changes in muscle metabolism, stress indices, dehydration and immune parameters, and body weight. These responses may increase disease susceptibility and influence energy availability for athletic performance following long-term transport of horses.